[PowerPC] Fix single-use check and update chain users for ld-splat

When converting a BUILD_VECTOR or VECTOR_SHUFFLE to a splatting load
as of 1461fb6e, we inaccurately check
for a single user of the load and neglect to update the users of the
output chain of the original load. As a result, we can emit a new
load when the original load is kept and the new load can be reordered
after a dependent store. This patch fixes those two issues.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47891