[llvm] [SPIR-V][Codegen] Add isPhi bit to MCInstrDesc/MIR definitions (PR #110019)

[Matt Arsenault via llvm-commits]

================
@@ -187,6 +187,7 @@ enum Flag {
   Trap,
   VariadicOpsAreDefs,
   Authenticated,
+  Phi,
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arsenm wrote:

These bits are precious and it would be best to avoid spending one on this 

https://github.com/llvm/llvm-project/pull/110019