[mips] Always check that `shift and add` optimization is efficient.

The D45316 introduced the `shouldTransformMulToShiftsAddsSubs` function
to check that breaking down constant multiplications into a series
of shifts, adds, and subs is efficient. Unfortunately, this function
does not check maximum number of steps on all paths of the algorithm.
This patch fixes this bug.

Fix for PR41929.

Differential Revision: https://reviews.llvm.org/D62166

llvm-svn: 361606