R600: Use StructurizeCFGPass for non SI targets

StructurizeCFG pass allows to make complex cfg reducible ; it allows a lot of
shader from shadertoy (which exhibits complex control flow constructs) to works
correctly with respect to CFG handling (and allow us to detect potential bug in
other part of the backend).

We provide a cmd line argument to disable the pass for debug purpose.

Patch by: Vincent Lejeune

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
R600/SI: Mark the EXEC register as reserved

This prevents the machine verifier from complaining about uses of
an undefined physical register.

Reviewed-by: Vincent Lejeune<vljn at ovi.com>
R600/SI: Define a separate MIMG instruction for each possible output value type

During instruction selection, we rewrite the destination register
class for MIMG instructions based on their writemasks.  This creates
machine verifier errors since the new register class does not match
the register class in the MIMG instruction definition.

We can avoid this by defining different MIMG instructions for each
possible destination type and then switching to the correct instruction
when we change the register class.
R600/SI: Use -verify-machineinstrs for most tests

We can't enable the verifier for tests with SI_IF and SI_ELSE, because
these instructions are always followed by a COPY which copies their
result to the next basic block.  This violates the machine verifier's
rule that non-terminators can not folow terminators.

Reviewed-by: Vincent Lejeune<vljn at ovi.com>