R600/SI: Remove apparently dead code in legalizeOperands

No tests hit this, and I don't see any way a GlobalAddress
node would survive beyond lowering on SI. It it would, the
move should probably be inserted by selection.
R600/SI: Fix using wrong operand indices when commuting

No test since the current SIISelLowering::legalizeOperands
effectively hides this, and the general uses seem to only fire
on SALU instructions which don't have modifiers between
the operands.

When trying to use legalizeOperands immediately after
instruction selection, it now sees a lot more patterns
it did not see before which break on this.
R600/SI: Don't assert on exotic operand types

This needs a test, but I'm not sure if it is currently possible and
I originally hit it due to a bug. Right now the only global address
operands have no reason to be VALU instructions, although it
theoretically could be a problem.
R600/SI: Don't move operands that are required to be SGPRs

e.g. v_cndmask_b32 requires the condition operand be an SGPR.
If one of the source operands were an SGPR, that would be considered
the one SGPR use and the condition operand would be illegally moved.
R600/SI: Implement findCommutedOpIndices

The base implementation of commuteInstruction is used
in some cases, but it turns out this has been broken for a
long time since modifiers were inserted between the real operands.

The base implementation of commuteInstruction also fails on immediates,
which also needs to be fixed.
R600/SI: Partially move operand legalization to post-isel hook.

Disable the SGPR usage restriction parts of the DAG legalizeOperands.
It now should only be doing immediate folding until it can be replaced
later. The real legalization work is now done by the other
SIInstrInfo::legalizeOperands
R600/SI Allow same SGPR to be used for multiple operands

Instead of moving the first SGPR that is different than the first,
legalize the operand that requires the fewest moves if one
SGPR is used for multiple operands.

This saves extra moves and is also required for some instructions
which require that the same operand be used for multiple operands.
R600/SI: Move finding SGPR operand to move to separate function
R600/SI: Add a note about the order of the operands to div_scale
R600/SI: Add strict check lines to div_scale tests.

This has weird operand requirements so it's worthwhile
to have very strict checks for its operands.

Add different combinations of SGPR operands.