[RISCV] Prevent formation of shXadd(.uw) and add.uw if it prevents the use of addi.

If the outer add has an simm12 immediate operand we should prefer
it instead of materializing it in a register. This would guarantee
and extra instruction and temporary register. Since we don't check
one use on the shl or zext we might generate more instructions if
there is an additional user.