[X86] Simplify FixupBW sub_8bit_hi-related logic. NFC.
[X86] Simplify FixupBW sub_8bit_hi-related logic. NFC.

Instead of passing around sizes and asking for subregs, we can check
the subreg indices we care about: sub_8bit_hi and sub_8bit.

Differential Revision: http://reviews.llvm.org/D20006

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@268753 91177308-0d34-0410-b5e6-96231b3b80d8

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