Fixes: rust-lang#84561

This fixes the temporary regression introduced in rust-lang#84339 where the wasm
target uses `fpto{s,u}i` intrinsics but the codegen for those intrinsics
with the `+nontrapping-fptoint` LLVM feature wasn't very good (aka it
didn't use the wasm instruction). The fixes brought in here fix that and
also implement the second-to-last simd instruction in LLVM.

Use flex more consistently

Builds on rust-lang#84376, related to rust-lang#84354.

- Fully replaces `float: right` with `flex` on `.content .out-of-band`.
- Uses `flex` more consistently with existing usage (on `h3`, `h4`, etc.).

Tested on various widths to make sure the pages behave as before.

Vastly improves coverage spans for macros

Fixes: rust-lang#84561

This resolves problems where macros like `trace!(...)` would show zero coverage if tracing was disabled, and `assert_eq!(...)` would show zero coverage if the assertion did not fail, because only one coverage span was generated, for the branch.

This PR started with an idea that I could just drop branching blocks with same span as expanded macro. (See the fixed issue for more details.)

That did help, but it didn't resolve everything.

I also needed to add a span specifically for the macro name (plus `!`) to ensure the macro gets coverage even if it's internal expansion adds conditional branching blocks that are retained, and would otherwise drop the outer span. Now that outer span is _only_ the `(argument, list)`, which can safely be dropped now), because the macro name has its own span.

While testing, I also noticed the spanview debug output can cause an ICE on a function with no body. The
workaround for this is included in this PR (separate commit).

r? ```@tmandry```
cc? ```@wesleywiser```

Add HAS_RE_LATE_BOUND if there are bound vars

Fixes rust-lang#83737
Fixes rust-lang#84604

I can rename `HAS_RE_LATE_BOUND`, to something like `HAS_LATE_BOUND_VARS`.

r? ```@nikomatsakis```

Update LLVM for more wasm simd updates

This fixes the temporary regression introduced in rust-lang#84339 where the wasm
target uses `fpto{s,u}i` intrinsics but the codegen for those intrinsics
with the `+nontrapping-fptoint` LLVM feature wasn't very good (aka it
didn't use the wasm instruction). The fixes brought in here fix that and
also implement the second-to-last simd instruction in LLVM.

Be stricter about rejecting LLVM reserved registers in asm!

LLVM will silently produce incorrect code if these registers are used as operands.

cc ```@rust-lang/wg-inline-asm```

…vink

Minor grammar tweaks for readability to btree internals

I was reading through the btree implementation and I noticed some grammar that could be improved in Node.rs so here is what I think would be a minor improvement.

…ss-for-search-results, r=Nemo157

Remove unnecessary CSS rules for search results

Discovered that this was useless when working on rust-lang/docs.rs#1382.

r? ```@Nemo157```

…in-search-results, r=Nemo157

Remove unneeded bottom margin on search results

As you can see, there is still more than enough space at the bottom:

![Screenshot from 2021-04-29 11-26-57](https://user-images.githubusercontent.com/3050060/116530090-ea797800-a8dd-11eb-8eef-2288cf68e0d2.png)

r? `@Nemo157`