[GlobalIsel][AMDGPU] Changing legalize rule for G_{UADDO|UADDE|USUBO|USUBE|SADDE|SSUBE}

Generic add and sub with carry are now legalized in a way to explicitly calculate carry/borrow output. i.e
%6:_(s64), %7:_(s1) = G_UADDO %0, %1
becomes,
%13:_(s32), %14:_(s1) = G_UADDO %2, %4
%15:_(s32), %16:_(s1) = G_UADDE %3, %5, %14
%6:_(s64) = G_MERGE_VALUES %13(s32), %15(s32)
%7:_(s1) = G_ICMP intpred(ult), %6(s64), %1

Here G_MERGE and G_ICMP instructions are redundant for recalculating carry output. (Similar case for sub with borrow)
This change fix this.

Reviewed By: arsenm, #amdgpu

Differential Revision: https://reviews.llvm.org/D137932