[llvm] [AArch64] Use pattern to select bf16 fpextend (PR #137212)

[David Green via llvm-commits]

davemgreen wrote:

> It looks like this doesn't work, as the fpext/fpround are introduced during legalization which happens from the bottom up, so at the time the fpext is created the fpround doesn't exist yet.

Ah, OK thanks for checking. Considering this fixes the noneon codegen and most of the important cases are a single instruction it looks OK to me. LGTM

https://github.com/llvm/llvm-project/pull/137212