This extends the patterns for loading into the zeroth lane of a zero vector
from D144086 to SVE, which work in the same way as the existing patterns. Only
full length vectors are added here, not the narrower floating point vector
types.
A LDR will implicitly zero the rest of the vector, so vector_insert(zeros,
load, 0) can use a single load. This adds tablegen patterns for both scaled and
unscaled loads, detecting where we are inserting a load into the lower element
of a zero vector.
Differential Revision: https://reviews.llvm.org/D144086
