[CodeGen] Generate efficient assembly for freeze(poison) version of `mm*_cast*` intel...

[CodeGen] Generate efficient assembly for freeze(poison) version of `mm*_cast*` intel intrinsics

This patch makes the variants of `mm*_cast*` intel intrinsics that use `shufflevector(freeze(poison), ..)` emit efficient assembly.
(These intrinsics are planned to use `shufflevector(freeze(poison), ..)` after shufflevector's semantics update; relevant thread: D103874)

To do so, this patch

1. Updates `LowerAVXCONCAT_VECTORS` in X86ISelLowering.cpp to recognize `FREEZE(UNDEF)` operand of `CONCAT_VECTOR` in addition to `UNDEF`
2. Updates X86InstrVecCompiler.td to recognize `insert_subvector` of `FREEZE(UNDEF)` vector as its first operand.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D130339