[LSR] Add test for implicit truncation on icmp immediate (NFC)

Test case for:
llvm#171456 (comment)

Author: nikic

llvm/test/Transforms/LoopStrengthReduce/X86/icmp-zero-offset-overflow.ll

@@ -0,0 +1,48 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -S -loop-reduce < %s | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @use(i1)
+
+; Make sure this does not assert.
+define i16 @test(i16 %start) #0 {
+; CHECK-LABEL: define i16 @test(
+; CHECK-SAME: i16 [[START:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    [[TMP0:%.*]] = sub i16 0, [[START]]
+; CHECK-NEXT:    [[TMP1:%.*]] = zext i16 [[TMP0]] to i64
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i64 [ [[LSR_IV_NEXT:%.*]], %[[LOOP]] ], [ 65536, %[[ENTRY]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = add i64 [[TMP1]], [[LSR_IV]]
+; CHECK-NEXT:    [[TMP:%.*]] = trunc i64 [[TMP2]] to i16
+; CHECK-NEXT:    [[IV2_CMP:%.*]] = icmp ne i16 [[TMP]], 0
+; CHECK-NEXT:    call void @use(i1 [[IV2_CMP]])
+; CHECK-NEXT:    [[LSR_IV_NEXT]] = add nsw i64 [[LSR_IV]], -1
+; CHECK-NEXT:    [[IV1_CMP:%.*]] = icmp eq i64 [[LSR_IV_NEXT]], 65532
+; CHECK-NEXT:    br i1 [[IV1_CMP]], label %[[EXIT:.*]], label %[[LOOP]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    [[TMP3:%.*]] = zext i16 [[START]] to i64
+; CHECK-NEXT:    [[TMP4:%.*]] = sub i64 [[TMP3]], [[LSR_IV_NEXT]]
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 [[TMP4]] to i16
+; CHECK-NEXT:    ret i16 [[TMP1]]
+;
+entry:
+  br label %loop
+
+loop:
+  %iv1 = phi i64 [ 3, %entry ], [ %iv1.dec, %loop ]
+  %iv2 = phi i16 [ %start, %entry ], [ %iv2.inc, %loop ]
+  %iv2.inc = add i16 %iv2, 1
+  %iv2.cmp = icmp ne i16 %iv2, 0
+  call void @use(i1 %iv2.cmp)
+  %iv1.dec = add i64 %iv1, -1
+  %iv1.cmp = icmp eq i64 %iv1, 0
+  br i1 %iv1.cmp, label %exit, label %loop
+
+exit:
+  ret i16 %iv2.inc
+}
+
+attributes #0 = { "target-cpu"="x86-64" }