[SVE] Relax merge requirement for IR based divides.

We currently lower SDIV to SDIV_MERGE_OP1. This forces the value
for inactive lanes in a way that can hamper register allocation,
however, the lowering has no requirement for inactive lanes.

Instead this patch replaces SDIV_MERGE_OP1 with SDIV_PRED thus
freeing the register allocator. Once done the only user of
SDIV_MERGE_OP1 is intrinsic lowering so I've removed the node
and perform ISel on the intrinsic directly. This also allows
us to implement MOVPRFX based zeroing in the same manner as SUB.

This patch also renames UDIV_MERGE_OP1 and [F]ADD_MERGE_OP1 for
the same reason but in the ADD cases the ISel code is already
as required.

Differential Revision: https://reviews.llvm.org/D82783