[ARM] Allow SP in rGPR, starting from ARMv8

Summary:
This patch handles assembly and disassembly, but not codegen, as of yet.

Additionally, it fixes a bug whereby SP and PC as shifted-reg operands
were treated as predictable in ARMv7 Thumb; and it enables the tests
for invalid and unpredictable instructions to run on both ARMv7 and ARMv8.

Reviewers: jmolloy, rengolin

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D14141

llvm-svn: 251516