[llvm] [AMDGPU][GlobalISel] Guard against use of disallowed sub-registers (PR #188781)

[Matt Arsenault via llvm-commits]

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@@ -5163,6 +5163,19 @@ static bool isSubRegOf(const SIRegisterInfo &TRI,
          SubReg.getReg() == SuperVec.getReg();
 }
 
+/// Check if given MachineOperand is an SGPR with SubReg hi16.
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arsenm wrote:

This shouldn't require manual verification. This should be implied by the register class and subregister definitions 

https://github.com/llvm/llvm-project/pull/188781