JIT: Account for mixed-enregistered locals when zeroing without block-init (#104593)

Locals that are in registers at the beginning of a function do not need
to have their stack home zeroed. `genFnProlog` already skips these
locals when computing the range of bytes to zero; however,
`genZeroInitFrame` was not doing the same in the non-block init case,
which does not make use of the range computed by `genFnProlog`. This
could cause an unbounded amount of (unnecessary) codegen during
zero-initing, which is not legal to have in the prolog.

Fix #104570

Author: jakobbotsch

src/coreclr/jit/codegencommon.cpp

@@ -4055,9 +4055,15 @@ void CodeGen::genZeroInitFrame(int untrLclHi, int untrLclLo, regNumber initReg,
                 continue;
             }
 
-            // TODO-Review: I'm not sure that we're correctly handling the mustInit case for
-            // partially-enregistered vars in the case where we don't use a block init.
-            noway_assert(varDsc->lvIsInReg() || varDsc->lvOnFrame);
+            // Locals that are (only) in registers to begin with do not need
+            // their stack home zeroed. Their register will be zeroed later in
+            // the prolog.
+            if (varDsc->lvIsInReg() && !varDsc->lvLiveInOutOfHndlr)
+            {
+                continue;
+            }
+
+            noway_assert(varDsc->lvOnFrame);
 
             // lvMustInit can only be set for GC types or TYP_STRUCT types
             // or when compInitMem is true
@@ -4066,11 +4072,6 @@ void CodeGen::genZeroInitFrame(int untrLclHi, int untrLclLo, regNumber initReg,
             noway_assert(varTypeIsGC(varDsc->TypeGet()) || (varDsc->TypeGet() == TYP_STRUCT) ||
                          compiler->info.compInitMem || compiler->opts.compDbgCode);
 
-            if (!varDsc->lvOnFrame)
-            {
-                continue;
-            }
-
             if ((varDsc->TypeGet() == TYP_STRUCT) && !compiler->info.compInitMem &&
                 (varDsc->lvExactSize() >= TARGET_POINTER_SIZE))
             {