[llvm] [GlobalISel] Introduce `G_POISON` (PR #127825)

[Matt Arsenault via llvm-commits]

================
@@ -11,3 +11,14 @@ define <4 x i1> @foo() {
 entry:
   ret <4 x i1> undef ;
 }
+
+define <4 x i1> @foo_poison() {
----------------
arsenm wrote:

Better to move this to a more obviously related test file, either an irtranslator-poison.ll or test/CodeGen/GlobalISel/arm64-irtranslator.ll is used as the catch-all 

https://github.com/llvm/llvm-project/pull/127825