New ARM Peephole optimizer FoldShiftLdrStr

This one folds
      mov r1, r2, lsl #2
      ldr/ldrb r0, [r0, r1]
into
      ldr/ldrb r0, [r0, r2, lsl #2]

There is still some room for improvement, maybe it would be better to do this before
the register allocator runs, as we'll currently waste a register (r1 in the above example)
in many cases. That would also allow to to fold more operations, because currently if r2
gets reused between the mov and ldr we'll not be able to do the optimization.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@23408 3ad0048d-3df7-0310-abae-a5850022a9f2