[InstCombine] Fold (select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C, Idx, 0)) (PR50183)

As discussed on PR50183, we already fold to prefer 'select-of-idx' vs 'select-of-gep':

define <4 x i32>* @select0a(<4 x i32>* %a0, i64 %a1, i1 %a2, i64 %a3) {
  %gep0 = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %a1
  %gep1 = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %a3
  %sel = select i1 %a2, <4 x i32>* %gep0, <4 x i32>* %gep1
  ret <4 x i32>* %sel
}
-->
define <4 x i32>* @select1a(<4 x i32>* %a0, i64 %a1, i1 %a2, i64 %a3) {
  %sel = select i1 %a2, i64 %a1, i64 %a3
  %gep = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %sel
  ret <4 x i32>* %gep
}

This patch adds basic handling for the 'fallthrough' cases where the gep idx == 0 has been folded away to the base address:

define <4 x i32>* @select0(<4 x i32>* %a0, i64 %a1, i1 %a2) {
  %gep = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %a1
  %sel = select i1 %a2, <4 x i32>* %a0, <4 x i32>* %gep
  ret <4 x i32>* %sel
}
-->
define <4 x i32>* @select1(<4 x i32>* %a0, i64 %a1, i1 %a2) {
  %sel = select i1 %a2, i64 0, i64 %a1
  %gep = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %sel
  ret <4 x i32>* %gep
}

Differential Revision: https://reviews.llvm.org/D105901

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