X86: Use enums for memory operand decoding instead of integer literals.

Summary:
X86BaseInfo.h defines an enum for the offset of each operand in a memory operand
sequence.  Some code uses it and some does not.  This patch replaces (hopefully)
all remaining locations where an integer literal was used instead of this enum.
No functionality change intended.

Reviewers: nadav

CC: llvm-commits, t.p.northover

Differential Revision: http://llvm-reviews.chandlerc.com/D3108