R600: Fix tracking of implicit defs in the IndirectAddressing pass

In some cases, we were losing track of live implicit registers which
was creating dead defs and causing the scheduler to produce invalid
code.

NOTE: This is a candidate for the Mesa stable branch.
R600: Fix scheduler crash caused by invalid MachinePointerInfo

Kernel function arguments are lowered to loads from the PARAM_I address
space.  When creating these load instructions, we were initializing
their MachinePointerInfo with an Arguement object that was not attached
to any function.  This was causing the MachineScheduler to crash when
it tried to access the parent of the Arguement.

This has been fixed by initializing the MachinePointerInfo with a
UndefValue instead.

NOTE: This is a candidate for the Mesa stable branch.
R600: Mark all members of the TRegMem register class as reserved

This stops the Machine Verifier from complaining about uses of undefined
physical registers.

NOTE: This is a candidate for the Mesa stable branch.
R600: Add AR_X to the R600_TReg_X register class.

NOTE: This is a candidate for the Mesa stable branch.
ARM NEON: Merge a f32 bitcast of a v2i32 extractelt

A vectorized sitfp on doubles will get scalarized to a sequence of an
extract_element of <2 x i32>, a bitcast to f32 and a sitofp.
Due to the the extract_element, and the bitcast we will uneccessarily generate
moves between scalar and vector registers.

The patch fixes this by using a COPY_TO_REGCLASS and a EXTRACT_SUBREG to extract
the element from the vector instead.

radar://13191881