Revert "[X86] Use push-pop for materializing small constants under 'minsize'"

Revert "[X86] Use push-pop for materializing small constants under 'minsize'"

The red zone consists of 128 bytes beyond the stack pointer so that the
allocation of objects in leaf functions doesn't require decrementing
rsp.  In r255656, we introduced an optimization that would cheaply
materialize certain constants via push/pop.  Push decrements the stack
pointer and stores it's result at what is now the top of the stack.
However, this means that using push/pop would encroach on the red zone.
PR26023 gives an example where this corrupts an object in the red zone.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256808 91177308-0d34-0410-b5e6-96231b3b80d8