arch-gcn3: Add case to op selector when operand is vcc_hi

Change-Id: Ib8846656e18aad04ccb8c9112bc629c69078fe36
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29971
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>

src/arch/gcn3/registers.cc

@@ -141,6 +141,8 @@ namespace Gcn3ISA
              *
              */
             regIdx = numScalarRegs - 2;
+        } else if (idx == REG_VCC_HI) {
+            regIdx = numScalarRegs - 1;
         } else if (idx == REG_FLAT_SCRATCH_LO) {
             /**
              * the FLAT_SCRATCH register occupies the two SRF entries