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

Use the 3-byte (4 with REX prefix) push-pop sequence for materializing
small constants. This is smaller than using a mov (5, 6 or 7 bytes
depending on size and REX prefix), but it's likely to be slower, so
only used for 'minsize'.

This is a follow-up to r255656.

Differential Revision: http://reviews.llvm.org/D15549

llvm-svn: 255936