[llvm] optimize `is_finite` assembly (PR #169402)

[Matt Arsenault via llvm-commits]

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arsenm wrote:

Test vector cases, half, float and double. Maybe bfloat too? Also can you add a test to a target that requires soft-fp expansion of the fsub and where half is illegal? I think this code was finicky about emitting legal types 

https://github.com/llvm/llvm-project/pull/169402