[llvm] [LV] Return Invalid from getLegacyCost when instruction cost forced. (PR #154543)

[Kerry McLaughlin via llvm-commits]

================
@@ -0,0 +1,24 @@
+; REQUIRES: asserts
+; RUN: opt < %s -passes=loop-vectorize -force-target-instruction-cost=1 -debug-only=loop-vectorize -S -disable-output 2>&1 | FileCheck %s
----------------
kmclaughlin-arm wrote:

I've moved the test into `force-target-instruction-cost.ll` and changed the flag in that test to `-force-target-instruction-cost=100` as suggested.

https://github.com/llvm/llvm-project/pull/154543