[X86] Simplify some stuff in X86DisassemblerDecoder. NFC

- Deciding that insn->sibIndex is SIB_INDEX_NONE does not require another
check beyond the fully decoded bits being equal to 0x4.
The expression insn->sibIndex == SIB_INDEX_sib could not have been true unless
index were 0x4, because SIB_INDEX_sib is merely the range base (SIB_INDEX_EAX)
plus 4. Respectively SIB_INDEX_sib64.

- Don't use a switch statement to perform left-shift.

Differential Revision: http://reviews.llvm.org/D9762

llvm-svn: 240598