[DAGCombiner] sink target-supported FP<->int cast op after concat vectors

[DAGCombiner] sink target-supported FP<->int cast op after concat vectors

Try to combine N short vector cast ops into 1 wide vector cast op:
concat (cast X), (cast Y)... -> cast (concat X, Y...)

This is part of solving PR45794:
https://bugs.llvm.org/show_bug.cgi?id=45794

As noted in the code comment, this is uglier than I was hoping because
the opcode determines whether we pass the source or destination type
to isOperationLegalOrCustom(). Also IIUC, there's no way to validate
what the other (dest or src) type is. Without the extra legality check
on that, there's an ARM regression test in:
test/CodeGen/ARM/isel-v8i32-crash.ll
...that will crash trying to lower an unsupported v8f32 to v8i16.

Differential Revision: https://reviews.llvm.org/D79360