[AMDGPU] Allow flexible register names in inline asm constraints

Currently AMDGPU inline asm only allow v and s as register names in constraints.

This patch allows the following register names in constraints: (n, m is unsigned integer, n < m)

v

s

{vn} or {v[n]}

{sn} or {s[n]}

{S} , where S is a special register name

{v[n:m]}

{s[n:m]}

Differential Revision: https://reviews.llvm.org/D37568

llvm-svn: 314452