Fixes for missing_asserts_for_indexing

[vishruth-thimmaiah]

This PR fixes issues with the missing_asserts_for_indexing lint.

• false positive when the first index is the highest(or equal) value in a list of indexes:

pub fn foo(slice: &[i32]) -> i32{
	slice[1] * slice[0]
}

• false negative when an assert statement if found after the indexing operation.

pub fn bar(slice: &[i32]) -> i32 {
	let product = slice[0] * slice[1];
	assert!(slice.len() > 1);
	product
}

examples: https://godbolt.org/z/s7Y47vKdE

closes: #14079

changelog: [missing_asserts_for_indexing]: ignore asserts found after indexing, and do not require asserts if the first index is highest.

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[llogiq]

Thank you!

[samueltardieu]

Requeued for merging, patch state is clean.