[InstCombine] Only fold extract element to trunc if vector `hasOneUse` (#115627)

This fixes a missed optimization caused by the `foldBitcastExtElt`
pattern interfering with other combine patterns. In the case I was
hitting, we have IR that combines two vectors into a new larger vector
by extracting elements and inserting them into the new vector.

```llvm
define <4 x half> @bitcast_extract_insert_to_shuffle(i32 %a, i32 %b) {
  %avec = bitcast i32 %a to <2 x half>
  %a0 = extractelement <2 x half> %avec, i32 0
  %a1 = extractelement <2 x half> %avec, i32 1
  %bvec = bitcast i32 %b to <2 x half>
  %b0 = extractelement <2 x half> %bvec, i32 0
  %b1 = extractelement <2 x half> %bvec, i32 1
  %ins0 = insertelement <4 x half> undef, half %a0, i32 0
  %ins1 = insertelement <4 x half> %ins0, half %a1, i32 1
  %ins2 = insertelement <4 x half> %ins1, half %b0, i32 2
  %ins3 = insertelement <4 x half> %ins2, half %b1, i32 3
  ret <4 x half> %ins3
}
```

With the current behavior, `InstCombine` converts each vector extract
sequence to

```llvm
  %tmp = trunc i32 %a to i16
  %a0 = bitcast i16 %tmp to half
  %a1 = extractelement <2 x half> %avec, i32 1
```

where the extraction of `%a0` is now done by truncating the original
integer. While on it's own this is fairly reasonable, in this case it
also blocks the pattern which converts `extractelement` -
`insertelement` into shuffles which gives the overall simpler result:

```llvm
define <4 x half> @bitcast_extract_insert_to_shuffle(i32 %a, i32 %b) {
  %avec = bitcast i32 %a to <2 x half>
  %bvec = bitcast i32 %b to <2 x half>
  %ins3 = shufflevector <2 x half> %avec, <2 x half> %bvec, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
  ret <4 x half> %ins3
}
```

In this PR I fix the conflict by obeying the `hasOneUse` check even if
there is no shift instruction required. In these cases we can't remove
the vector completely, so the pattern has less benefit anyway.

Also fwiw, I think dropping the `hasOneUse` check for the 0th element
might have been a mistake in the first place. Looking at
https://github.com/llvm/llvm-project/commit/535c5d56a7bc9966036a11362d8984983a4bf090
the commit message only mentions loosening the `isDesirableIntType`
requirement and doesn't mention changing the `hasOneUse` check at all.