Improve reduction intrinsics by overloading result value.

This patch uses the mechanism from D62995 to strengthen the
definitions of the reduction intrinsics by letting the scalar
result/accumulator type be overloaded from the vector element type.

For example:

  ; The LLVM LangRef specifies that the scalar result must equal the
  ; vector element type, but this is not checked/enforced by LLVM.
  declare i32 @llvm.experimental.vector.reduce.or.i32.v4i32(<4 x i32> %a)

This patch changes that into:

  declare i32 @llvm.experimental.vector.reduce.or.v4i32(<4 x i32> %a)

Which has the type-constraint more explicit and causes LLVM to check
the result type with the vector element type.

Reviewers: RKSimon, arsenm, rnk, greened, aemerson

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D62996

llvm-svn: 363240