[llvm] [CodeGenTypes] Speed up getVectorElementType and getVectorMinNumElements (PR #95282)

[Jay Foad via llvm-commits]

jayfoad wrote:

> This speeds up "check-llvm-codegen-amdgpu" by about 7% in my Release build.

The reason it has such a large effect on the AMDGPU test suite is that a huge number of those tests use `-verify-machineinstrs`, and when MachineVerifier verifies a COPY instruction it calls `TRI->getMinimalPhysRegClassLLT` on both operands.

https://github.com/llvm/llvm-project/pull/95282