[LoongArch] Lower vector select mask generation to `[X]VMSK{LT,GE,NE}Z` if possible (#142109)
This patch adds a DAG combine rule for BITCAST nodes converting from
vector `i1` masks generated by `setcc` into integer vector types. It
recognizes common select mask patterns and lowers them into efficient
LoongArch LSX/LASX mask instructions such as:
- [X]VMSKLTZ.{B,H,W,D}
- [X]VMSKGEZ.B
- [X]VMSKNEZ.B
When the vector comparison matches specific patterns (e.g., x < 0, x >=
0, x != 0, etc.), the transformation is performed pre-legalization. This
avoids scalarization and unnecessary operations, improving both
performance and code size.
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