JIT: Skip GC info for trivial async calls for x86 (#121941)

For x86 we only report GC pointers live in callee save registers, and we
also avoid reporting any trivial call.
There is compensating code upstream of the GC reporting that returns for
trivial calls, but this code did not account for the GC ref returned for
async continuations. Thus it was possible that we made it into the GC
encoder with a trivial call and then hit an assert.

Fix #121872

Author: jakobbotsch

src/coreclr/jit/emit.cpp

@@ -10373,9 +10373,10 @@ void emitter::emitStackPopLargeStk(BYTE* addr, bool isCall, unsigned char callIn
     assert(regMaskTP::FromIntRegSet(SingleTypeRegSet(byrefRegs << REG_INT_FIRST)) == emitThisByrefRegs);
 
 #ifdef JIT32_GCENCODER
-    // x86 does not report GC refs/byrefs in return registers at call sites
-    gcrefRegs &= ~(1u << (REG_INTRET - REG_INT_FIRST));
-    byrefRegs &= ~(1u << (REG_INTRET - REG_INT_FIRST));
+    // x86 only reports GC refs/byrefs in callee saves at call sites -- no return registers.
+    unsigned reportedRegs = (RBM_INT_CALLEE_SAVED | RBM_EBP).GetIntRegSet() >> REG_INT_FIRST;
+    gcrefRegs &= reportedRegs;
+    byrefRegs &= reportedRegs;
 
     // For the general encoder, we always have to record calls, so we don't take this early return.    /* Are there any
     // args to pop at this call site?