ARM Assembly support for Thumb mov-immediate.

Correctly parse the forms of the Thumb mov-immediate instruction:
  1. 8-bit immediate 0-255.
  2. 12-bit shifted-immediate.

The 16-bit immediate "movw" form is also legal with just a "mov" mnemonic,
but is not yet supported. More parser logic necessary there due to fixups.

llvm-svn: 133966