[SelectionDAG] Use getRegisterType instead of getTypeToTransformTo in...

[SelectionDAG] Use getRegisterType instead of getTypeToTransformTo in ComputePHILiveOutRegInfo. (#80773)

Since we used getNumRegisters right before this, I think this is the
correct interface we should be using here.

I'm experimenting with making i32 legal on RISC-V 64, but using i64 for
the register type between basic blocks. This was one of the first issues
I found trying to do that.