[LegalizeTypes][RISCV] Add a special case for (add X, -1) to ExpandIntRes_ADDSUB

 On targets without ADDCARRY or ADDE, we need to emit a separate
 SETCC to determine carry from the low half to the high half.
 The high half is calculated by a series of ADDs.

 When RHSLo and RHSHi are -1, without this patch, we get:
   Hi = (add (add LHSHi,(setult Lo, LHSLo), -1)
 Where as with the patch we get:
   Hi = (sub LHSHi, (seteq LHSLo, 0))

 Only RHSLo is -1 we can instead do (setne Lo, 0).

 Similar to gcc: https://godbolt.org/z/M83f6rz39

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D146635