[llvm] [AMDGPU] Introduce and use NotUseRealTrue16Insts. NFC. (PR #161373)

[Joe Nash via llvm-commits]

https://github.com/Sisyph approved this pull request.

LGTM, but please give others a chance to comment.
In terms of fundamental modes of instructions, we need 3. NotHasTrue16BitInsts, UseFakeTrue16 and useRealTrue16. We have 3 versions of instructions that correspond to those. For patterns we need 2 fundamental modes, the true16 pattern and the other pattern (which is used for NotHasTrue16 and UseFakeTrue16). 
Then we can talk about having composite predicates on top of that, to make the Tablegen cleaner.
HasTrue16BitInsts is a composite of UseFakeTrue16 and useRealTrue16. This new predicate is a composite of NotUseRealTrue16Insts and NotHasTrue16BitInsts. 

https://github.com/llvm/llvm-project/pull/161373