[llvm] [LoongArch] Add DAG combines for vector widening extends (PR #207316)

[via llvm-commits]

heiher wrote:

Don't know how to legalize this operation
UNREACHABLE executed at /home/hev/git/llvm/llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp:5576!

```llvm
target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
target triple = "loongarch64-unknown-linux-gnu"

define void @xvexth_q_d(ptr %a, ptr %r) nounwind #0 {
entry:
  %0 = load <4 x i64>, ptr %a
  %1 = shufflevector <4 x i64> %0, <4 x i64> poison,
                     <2 x i32> <i32 1, i32 3>
  %2 = sext <2 x i64> %1 to <2 x i128>
  store <2 x i128> %2, ptr %r
  ret void
}

attributes #0 = { "target-features"="+relax,+64bit,+f,+d,+ual,+lsx,+lasx" }

```

https://github.com/llvm/llvm-project/pull/207316