Fix crash in JITIR after disassembly improvement.

Fixes #19292

Core/MIPS/IR/IRInst.cpp

@@ -17,6 +17,9 @@
 //  v = Vec4Init constant, chosen by immediate
 //  s = Shuffle immediate (4 2-bit fields, choosing a xyzw shuffle)
 //  r = Replacement function (in constant field)
+//
+//  WARNING: The IRJit compiler also uses these letters for semantic information!
+//  So if you add new letters, don't forget to add them to IRNativeRegCacheBase::MappingFromInst.
 
 static const IRMeta irMeta[] = {
 	{ IROp::Nop, "Nop", "" },

Core/MIPS/IR/IRRegCache.cpp

@@ -911,6 +911,7 @@ void IRNativeRegCacheBase::MappingFromInst(const IRInst &inst, Mapping mapping[3
 		case '\0':
 		case '_':
 		case 'C':
+		case 'r':
 		case 'I':
 		case 'v':
 		case 's':