runtime: PMI diffs are failing due to assert

jit-diff.bat diff --diff --pmi --frameworks currently fails for the following methods:

• PREPALL type# 1350 method# 20377 System.Text.Encoding::GetCharsWithFallback
• PREPALL type# 1350 method# 20378 System.Text.Encoding::GetCharsWithFallback
• PREPALL type# 1350 method# 20472 System.Text.Encoding::GetBytesWithFallback
• PREPALL type# 1350 method# 20473 System.Text.Encoding::GetBytesWithFallback
• PREPALL type# 1350 method# 20477 System.Text.Encoding::GetCharCountWithFallback
• PREPALL type# 1350 method# 20478 System.Text.Encoding::GetCharCountWithFallback

In all cases, an assert in emitOutputRR is firing: https://github.com/dotnet/runtime/blob/main/src/coreclr/jit/emitxarch.cpp#L11738-L11739

                    case INS_add:
                    case INS_sub:
                        assert(id->idGCref() == GCT_BYREF);

#ifdef DEBUG
                        regMaskTP regMask;
                        regMask = genRegMask(reg1) | genRegMask(reg2);

                        // r1/r2 could have been a GCREF as GCREF + int=BYREF
                        //                            or BYREF+/-int=BYREF
                        assert(((regMask & emitThisGCrefRegs) && (ins == INS_add)) ||
                               ((regMask & emitThisByrefRegs) && (ins == INS_add || ins == INS_sub)));
#endif
                        // Mark r1 as holding a byref
                        emitGCregLiveUpd(GCT_BYREF, reg1, dst);
                        break;