[llvm] [Hexagon] Fix fpround fp32 -> fp16 pattern (PR #199704)

[Nicolas Miller via llvm-commits]

npmiller wrote:

> Sure, which issue does this patch address?

I didn't file an issue for this, it's something I ran into in code doing casting from fp16 to fp32 and back to run an operation at higher precision, and I was getting incorrect results, I ended up tracking it down to this pattern.

https://github.com/llvm/llvm-project/pull/199704