[llvm] [AArch64][GlobalISel] More FCmp legalization. (PR #78734)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

> G_SELECT shows an optimization opportunity, if the LLT could distinguish between i16 and f16. i16 would never be legal and f16 would sometimes be legal for G_SELECT. The costs are that we have to legalize G_SELECT for i64 and f64.

Yeah that's what I was saying about "It is as if we need to do legalization -> regbankselect -> extra legaliation -> selection", at least in the current design without fp types. 

https://github.com/llvm/llvm-project/pull/78734