[clang] [clang] Fix ignoring converted first arg of elementwise_fma (PR #154941)

[Chaitanya Koparkar via cfe-commits]

ckoparkar wrote:

The fshl/r builtins also run into the same issue since they're ternary ops as well. I can confirm that this patch fixes that case as well.

https://github.com/llvm/llvm-project/pull/154941