Fix off-by-one errors in limit checking for ldr/str instructions.

The LDR/STR encoder in the thumb assembler had an off-by-one
error for limit checking for immediates.  This resulted in an
assertion failure for things like 'ldr rx,[ry,#128]'

Bug: 15876206

Change-Id: Ic866212e2feae94e0bd4c753724898d84f5cb944
3 files changed