Lift the enums out of IceInstX8632.h and IceTargetLoweringX8632.h.
This will later allow the assembler to share the enum values and
use them as encodings where appropriate. E.g., to avoid having a separate
enum in:
https://codereview.chromium.org/476323004/diff/680001/src/assembler_constants_ia32.h

The "all registers" enum is retained, but separate GPRRegister and
XmmRegister enums are created with tags "Encoded_Reg_foo" to represent
the encoded value of register "foo". Functions are added to convert
from the "all registers" namespace to the encoded ones.

Re-order the BrCond so that they match the encoding according to
the "Instruction Subcode" in B.1 of the Intel Manuals.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/582113003