[X86] Fix miscompile in combineShiftRightArithmetic

When folding (ashr (shl, x, c1), c2) we need to treat c1 and c2
as unsigned to find out if the combined shift should be a left
or right shift.
Also do an early out during pre-legalization in case c1 and c2
has different types, as that otherwise complicated the comparison
of c1 and c2 a bit.

(cherry picked from commit 3e6e54eb)