[X86] Improve illegal return type handling in FastISel (#186723) Previously, FastISel would fall back to DAG ISel for any illegal return type. This change adds a more precise check to determine if the ABI requires a type conversion that FastISel cannot handle. For example, bfloat is returned as f16 in XMM0, but FastISel would assign f32 register type and store it in FuncInfo.ValueMap, causing DAG to incorrectly perform type conversion from f32 to bfloat later. However, i1 is promoted to i8 and returned as i8 per the ABI, so FastISel can safely lower it without switching to DAGISel. This change enables FastISel to handle such cases properly. --------- Co-authored-by: Yuanke Luo <ykluo@birentech.com> (cherry picked from commit 140adc92d509eeba74dc9d67d8d7bfdf1c485fc7)
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