[llvm] [SandboxIR] Implement PHINodes (PR #101111)

[via llvm-commits]

================
@@ -1290,6 +1292,97 @@ class GetElementPtrInst final : public Instruction {
 #endif
 };
 
+class PHINode final : public Instruction {
+  /// Use Context::createPHINode(). Don't call the
----------------
vporpo wrote:

This comment looks like it should fit in a single 80-column line.

https://github.com/llvm/llvm-project/pull/101111