[AMDGPU] Fixed isLegalRegOperand() with physregs

This does not change anything at the moment, but needed for
D89170. In that change I am probing a physical SGPR to see if
it is legal. RC is SReg_32, but DRC for scratch instructions
is SReg_32_XEXEC_HI and test fails.

That is sufficient just to check if DRC contains a register
here in case of physreg. Physregs also do not use subregs
so the subreg handling below is irrelevant for these.

Differential Revision: https://reviews.llvm.org/D90064