[InstCombine] Fold extractelement + vector GEP with one use

We sometimes see code like this:

Case 1:
  %gep = getelementptr i32, i32* %a, <2 x i64> %splat
  %ext = extractelement <2 x i32*> %gep, i32 0

or this:

Case 2:
  %gep = getelementptr i32, <4 x i32*> %a, i64 1
  %ext = extractelement <4 x i32*> %gep, i32 0

where there is only one use of the GEP. In such cases it makes
sense to fold the two together such that we create a scalar GEP:

Case 1:
  %ext = extractelement <2 x i64> %splat, i32 0
  %gep = getelementptr i32, i32* %a, i64 %ext

Case 2:
  %ext = extractelement <2 x i32*> %a, i32 0
  %gep = getelementptr i32, i32* %ext, i64 1

This may create further folding opportunities as a result, i.e.
the extract of a splat vector can be completely eliminated. Also,
even for the general case where the vector operand is not a splat
it seems beneficial to create a scalar GEP and extract the scalar
element from the operand. Therefore, in this patch I've assumed
that a scalar GEP is always preferrable to a vector GEP and have
added code to unconditionally fold the extract + GEP.

I haven't added folds for the case when we have both a vector of
pointers and a vector of indices, since this would require
generating an additional extractelement operation.

Tests have been added here:

  Transforms/InstCombine/gep-vector-indices.ll

Differential Revision: https://reviews.llvm.org/D101900