[LoongArch] Support memcmp expansion for vectors and combine for i128/i256 setcc
This commit enables memcmp expansion for lsx/lasx. After doing
this, i128 and i256 loads which are illegal types on LoongArch
will be generated. Without process, they will be splited to
legal scalar type.
So this commit also enable combination for `setcc` to bitcast
i128/i256 types to vector types before type legalization and
generate vector instructions.
Inspired by x86 and riscv.