[AMDGPU][llvm-mc] s_getreg/setreg* - Add hwreg(...) syntax.

Added hwreg(reg[,offset,width]) syntax.
Default offset = 0, default width = 32.
Possibility to specify 16-bit immediate kept.
Added out-of-range checks.
Disassembling is always to hwreg(...) format.
Tests updated/added.

Differential Revision: http://reviews.llvm.org/D19329

llvm-svn: 267410