ARM: use litpools for normal i32 imms when compiling minsize.

With constant-sharing, litpool loads consume 4 + N*2 bytes of code, but
movw/movt pairs consume 8*N. This means litpools are better than movw/movt even
with just one use. Other materialisation strategies can still be better though,
so the logic is a little odd.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199891 91177308-0d34-0410-b5e6-96231b3b80d8