ARM: rework Thumb1 frame index rewriting

The previous code had a few problems, motivating the choices here.

1. It could create instructions clobbering CPSR, but the incoming MachineInstr
   didn't reflect this. A potential source of corruption. This is why the patch
   has a new PseudoInst for before lowering.
2. Similarly, there was some code to handle the incoming instruction not being
   ARMCC::AL, but this would have caused massive problems if it was actually
   invoked when a complex offset needing more than one instruction was requested.
3. It wasn't designed to handle unaligned pointers (or offsets). These should
   probably be minimised anyway, but the code needs to deal with them properly
   regardless.
4. It had some rather dubious ad-hoc code to avoid calling
   emitThumbRegPlusImmediate, a function which should be designed to do precisely
   this job.

We seem to cover the common cases correctly now, and hopefully can enhance
emitThumbRegPlusImmediate to handle any extra optimisations we need to add in
future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220236 91177308-0d34-0410-b5e6-96231b3b80d8
Explain why we don't always use --gc-sections.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220237 91177308-0d34-0410-b5e6-96231b3b80d8
[X86] Memory folding for commutative instructions (updated)

This patch improves support for commutative instructions in the x86 memory folding implementation by attempting to fold a commuted version of the instruction if the original folding fails - if that folding fails as well the instruction is 're-commuted' back to its original order before returning.

Updated version of r219584 (reverted in r219595) - the commutation attempt now explicitly ensures that neither of the commuted source operands are tied to the destination operand / register, which was the source of all the regressions that occurred with the original patch attempt.

Added additional regression test case provided by Joerg Sonnenberger.

Differential Revision: http://reviews.llvm.org/D5818



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220239 91177308-0d34-0410-b5e6-96231b3b80d8