SelectionDAG: Teach FoldConstantArithmetic how to deal with vectors.

This required disabling a PowerPC optimization that did the following:
input:
x = BUILD_VECTOR <i32 16, i32 16, i32 16, i32 16>
lowered to:
tmp = BUILD_VECTOR <i32 8, i32 8, i32 8, i32 8>
x = ADD tmp, tmp

The add now gets folded immediately and we're back at the BUILD_VECTOR we
started from. I don't see a way to fix this currently so I left it disabled
for now.

Fix some trivially foldable X86 tests too.
X86: Simplify code. No functionality change.
X86: Open up some opportunities for constant folding by postponing shift lowering.

Fixes PR15141.