[IRBuilder] Use canonical i64 type for insertelement index used by vector splats.

Instcombine prefers this canonical form (see getPreferredVectorIndex),
as does IRBuilder when passing the index as an integer so we may as
well use the prefered form from creation.

NOTE: All test changes are mechanical with nothing else expected
beyond a change of index type from i32 to i64.

Differential Revision: https://reviews.llvm.org/D140983