[X86] Fix incorrect/inefficient pushw encodings for x86-64 targets

Correctly support assembling "pushw $imm8" on x86-64 targets. 
Also some cleanup of the PUSH instructions (PUSH64i16 and PUSHi16 actually
represent the same instruction)

This fixes PR23996

Patch by: david.l.kreitzer@intel.com
Differential Revision: http://reviews.llvm.org/D10878

llvm-svn: 241404