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 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241404 91177308-0d34-0410-b5e6-96231b3b80d8