(sub X, imm) gets canonicalized to (add X, -imm)

There are patterns to handle immediates when they fit in the immediate field.
e.g. %sub = add i32 %x, -123
=>   sub r0, r0, #123
Add patterns to catch immediates that do not fit but should be materialized
with a single movw instruction rather than movw + movt pair.
e.g. %sub = add i32 %x, -65535
=>   movw r1, #65535
     sub r0, r0, r1

rdar://11726136

llvm-svn: 159057