[llvm-reduce] Fix crash when reducing integer vectors to 1

Integer vectors were previously ignored when reducing operands. When
6b8bd0f7 introduced support for reducing floating-point
scalars/vectors, the vector case was written to only handle
floating-point values. It would crash when creating an invalid
ConstantFP from the integer element type.

Instead of reinstating the old integer vector behaviour, we might as
well reduce integer vectors to all-one splats.

A couple of existing tests has also been renamed from "remove" to
"reduce" to better reflect the deltas they test.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D129629