Add support for emitting both signed- and zero-extend loads. Fix

SimplifyAddress to handle either a 12-bit unsigned offset or the ARM +/-imm8
offsets (addressing mode 3).  This enables a load followed by an integer 
extend to be folded into a single load.

For example:
ldrb r1, [r0]       ldrb r1, [r0]
uxtb r2, r1     =>
mov  r3, r2         mov  r3, r1

llvm-svn: 144488