[LV] Exclude loop-invariant inputs from scalar cost computation.

Loop invariant operands do not need to be scalarized, as we are using
the values outside the loop. We should ignore them when computing the
scalarization overhead.

Fixes PR41294

Reviewers: hsaito, rengolin, dcaballe, Ayal

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D59995

llvm-svn: 366030