Get rid of all references to soimm in MipsConstantIslands pass because

we don't have such an operand.
Suprisingly enough, this is never actually accounted for in the 
ARM version when determining offset ranges. In both places there is the
comment:
-    // FIXME: Make use full range of soimm values.
(soimm = shift operand immediate).

llvm-svn: 194101