[ARM] Don't expand sdiv when optimising for minsize

Don't expand SDIV with an immediate that is a power of 2 if we optimise for
minimum code size. For example:

sdiv %1, i32 4

gets expanded to a sequence of 3 instructions, but this is suboptimal for
minimum code size so instead we just generate a MOV and a SDIV if integer
division is supported.

Differential Revision: https://reviews.llvm.org/D54546

llvm-svn: 347965