[ARM] Issue an erorr when non-general-purpose registers are used in address operands

Currently the assembler would accept, e.g. `ldr r0, [s0, #12]` and similar.
This patch add checks that only general-purpose registers are used in address
operands, shifted registers, and shift amounts.

Differential revision: https://reviews.llvm.org/D39910

llvm-svn: 321866