[AArch64][SME] Create new pass to remove COALESCER_BARRIER early. (#85386)

The purpose of the COALESCER_BARRIER pseudo node is to prevent the
register coalescer from coalescing certain COPY instructions around
smstart/smstop instructions, so that we spill only the (required) FPR
register rather than the encompassing ZPR register.

The pseudos are removed in the AArch64ExpandPseudo pass. However,
because the node itself is a _use_ of a register, this occassionally
leads to redundant spills/fills, because the register allocator thinks
the virtual register is actually used before an smstart/smstop
instruction, causing it to be filled, at which points it requires
immediate spilling again to ensure it stays live over the smstart/smstop
instruction.

We can avoid that by removing the pseudo nodes right after coalescing,
but before register allocation.