Add two GNU extension forms of wrpr:
wrpr %reg, %rd -> wrpr %reg, %g0, %rd
wrpr imm, %rd -> wrpr %g0, imm, %rd
Note: since the semantics of wrpr %rs1, %rs2/imm, %rd is %rd <- (%rs1 xor %rs2/imm),
in case (1) it technically doesn't matter whether the %g0 is placed as the first or
second operand, however, I'm placing it as the second operand to match the encoding
of GNU as.
This fixes the wrpr instruction part of bug #40792
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D139535
b34bfc603c