[llvm] [RISCV] Use implicit def/use of SP for PROBED_STACKALLOC*. (PR #139153)

[Sergei Barannikov via llvm-commits]

================
@@ -1452,17 +1452,18 @@ def GIAddrRegImm :
 
 /// Stack probing
 
-let hasSideEffects = 1, mayLoad = 1, mayStore = 1, isCodeGenOnly = 1 in {
+let hasSideEffects = 1, mayLoad = 1, mayStore = 1, isCodeGenOnly = 1,
+    Defs = [X2], Uses = [X2] in {
 // Probed stack allocation of a constant size, used in function prologues when
 // stack-clash protection is enabled.
-def PROBED_STACKALLOC : Pseudo<(outs GPR:$sp),
-                               (ins GPR:$scratch),
-                               []>,
-                               Sched<[]>;
-def PROBED_STACKALLOC_RVV : Pseudo<(outs GPR:$sp),
-                               (ins GPR:$scratch),
+def PROBED_STACKALLOC : Pseudo<(outs),
+                               (ins GPR:$target),
                                []>,
                                Sched<[]>;
+def PROBED_STACKALLOC_RVV : Pseudo<(outs),
+                                   (ins GPR:$target),
----------------
s-barannikov wrote:

(pedantic nit) Not sure it makes any difference, but from liveness point of view this should probably be:
```suggestion
let Constraints = "$scratch = $target" in
def PROBED_STACKALLOC_RVV : Pseudo<(outs GPR:$scratch),
                                   (ins GPR:$target),
```
since RVV expansion [modifies](https://github.com/llvm/llvm-project/blob/4e175b889669b430113ce0216e0ee1fe7884889c/llvm/lib/Target/RISCV/RISCVFrameLowering.cpp#L2373) the `$target` register.


https://github.com/llvm/llvm-project/pull/139153