R600: Unconditionally unroll loops that contain GEPs with alloca pointers

Implement the getUnrollingPreferences() function for
AMDGPUTargetTransformInfo so that loops that do address calculations
on pointers derived from alloca are unconditionally unrolled.

Unrolling these loops makes it more likely that SROA will be able to
eliminate the allocas, which is a big win for R600 since memory
allocated by alloca (private memory) is really slow.
R600: Correctly handle vertex fetch clauses the precede ENDIFs

The control flow finalizer would sometimes use an ALU_POP_AFTER
instruction before the vetex fetch clause instead of using a POP
instruction after it.
R600: Disable the BFE pattern

This pattern uses an SDNodeXForm, which isn't being emitted for some
reason.  I can get it to work by attaching the PatLeaf that has the
XForm to the argument in the output pattern, but this results in an
immediate being used in a register operand, which the backend can't
handle yet.
R600: Remove successive JUMP in AnalyzeBranch when AllowModify is true

This fixes a crash in the OpenCV OpenCL test suite.

There is no lit test for this, because the test would be very large
and could easily be invalidated by changes to the scheduler
or other parts of the compiler.

Patch by:  Vincent Lejeune