[LV] Use correct insertion point when type shrinking reductions

When type shrinking reductions, we should insert the truncations and extends at
the end of the loop latch block. Previously, these instructions were inserted
at the end of the loop header block. The difference is only a problem for loops
with predicated instructions (e.g., conditional stores and instructions that
may divide by zero). For these instructions, we create new basic blocks inside
the vectorized loop, which cause the loop header and latch to no longer be the
same block. This should fix PR34687.

Reference: https://bugs.llvm.org/show_bug.cgi?id=34687
llvm-svn: 314542