ARM: Fix ELF global base reg intialization.

The create machine code wasn't properly in SSA, which the machine verifier
properly complains about. Now that fast-isel is closer to verifier clean,
errors like this show up more clearly.

Additionally, the Thumb pseudo tPICADD was used for both ARM and Thumb
mode functions, which is obviously wrong. Fix that along the way.

Test case is part of the following commit which will finish making an
additional fast-isel test verifier clean an enable it for the
regression test suite. This commit is separate since its not just
a verifier cleanup, but an actual correctness issue.

rdar://12594152 (for the fast-isel verifier aspects)
ARM: FastISel verifier error cleanup.

Constant pool and global value reference instructions need more
restricted register classes than plain GPR.

rdar://12594152