[LegalizeTypes][ARM][AArch64][PowerPC][RISCV][X86] Use BUILD_PAIR to return expanded...

[LegalizeTypes][ARM][AArch64][PowerPC][RISCV][X86] Use BUILD_PAIR to return expanded integer results from ReplaceNodeResults instead of just returning two results.

Remove code from LegalizeTypes that allowed this to work.

We were already using BUILD_PAIR for this in some places so this
standardizes on a single way to do this.