ARM: constrain Thumb LDRLIT pseudo-instructions to r0-r7.

Previously we only used GPR for the destination placeholder in "ldr rD, [pc,
incorrect codegen under the integrated assembler.

This should fix both issues (which probably only affect MachO targets at the
moment).

rdar://problem/15800156

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199108 91177308-0d34-0410-b5e6-96231b3b80d8