Merging r300429:

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r300429 | d0k | 2017-04-16 16:13:08 -0400 (Sun, 16 Apr 2017) | 7 lines

[X86] Remove special handling for 16 bit for A asm constraints.

Our 16 bit support is assembler-only + the terrible hack that is
.code16gcc. Simply using 32 bit registers does the right thing for the
latter.

Fixes PR32681.
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llvm-svn: 301437