[AMDGPU] Fixed physreg asm constraint parsing

We are always failing parsing of the physreg constraint because
we do not drop trailing brace, thus getAsInteger() returns a
non-empty string and we delegate reparsing to the TargetLowering.

In addition it did not parse register tuples.

Fixed which has allowed to remove w/a in two places we call it.

Differential Revision: https://reviews.llvm.org/D117055