[LV] Fix PR34711 - widen instruction ranges when sinking casts

Instead of trying to keep LastWidenRecipe updated after creating each recipe,
have tryToWiden() retrieve the last recipe of the current VPBasicBlock and check
if it's a VPWidenRecipe when attempting to extend its range. This ensures that
such extensions, optimized to maintain the original instruction order, do so
only when the instructions are to maintain their relative order. The latter does
not always hold, e.g., when a cast needs to sink to unravel first order
recurrence (r306884).

Testcase derived from reproducer of PR34711.

Differential Revision: https://reviews.llvm.org/D38339

llvm-svn: 314981