Fix treatment of ARM unallocated hint instructions.

The reference manual defines only 5 permitted values for the immediate field of the "hint" instruction:
1. nop (imm == 0)
2. yield (imm == 1)
3. wfe (imm == 2)
4. wfi (imm == 3)
5. sev (imm == 4)

Therefore, restrict the permitted values for the "hint" instruction to 0 through 4.

Patch by Mihail Popa <Mihail.Popa@arm.com>

llvm-svn: 179707