[llvm] [GISEL] Add G_VSCALE instruction (PR #84542)

[Thorsten Schütt via llvm-commits]

================
@@ -0,0 +1,15 @@
+# RUN: not --crash llc -verify-machineinstrs -run-pass none -o /dev/null %s 2>&1 | FileCheck %s
+
+---
+name:            g_vscale
+body: |
+  bb.0:
+
+  %1:_(s32) = G_CONSTANT 4
+
+  ; CHECK: G_VSCALE operand must be cimm
+  %2:_(s32) = G_VSCALE %1
+
+  ; CHECK: G_VSCALE immediate cannot be zero
----------------
tschuett wrote:

G_CONSTANT slightly differs:
```
def G_CONSTANT : GenericInstruction {
  let OutOperandList = (outs type0:$dst);
  let InOperandList = (ins unknown:$imm);
  let hasSideEffects = false;
}
```
Note the `$imm`.

https://github.com/llvm/llvm-project/pull/84542