[ARM] Minor cleanup to lXX and stX. Fixes a bug.
Fixes an issue in lXX where if the instruction was an update instruction and was a register address offset that we would add garbage values to the
writeback register.
Does some minor cleanups as well which save a few instructions here and there.
Merge pull request #257 from Sonicadvance1/LoadStore-Fix

[ARM] Minor cleanup to lXX and stX. Fixes a bug.