[ARM] Convert test to opaque pointers

When converting this test to opaque pointers, we get a register
move between the call and the inline asm. However, the test
comment specifically says that there should be nothing between them.

As far as I can tell, this is fine, both in that the inline asm
doesn't use the relevant registers, but also more generally
because the inline asm doesn't declare any clobbers, so really
LLVM can do whatever, side effects or not. The test was added
by 618ce3e8 with only a reference
to Apple's internal issue tracker.

Differential Revision: https://reviews.llvm.org/D147512