DAG: fp->int conversion for non-splat constants.

Constant fold the lanes of the input constant build_vector individually
so we correctly handle when the vector elements are not all the same
constant value.

PR20394
X86: restrict combine to when type sizes are safe.

The folding of unary operations through a vector compare and mask operation
is only safe if the unary operation result is of the same size as its input.
For example, it's not safe for [su]itofp from v4i32 to v4f64.
[X86,AArch64] Extend vcmp w/ unary op combine to work w/ more constants.

The transform to constant fold unary operations with an AND across a
vector comparison applies when the constant is not a splat of a scalar
as well.