[InstCombine] Baseline test exposing vector fp-to-int conversion becoming scalar (#165800)
Baseline test for https://github.com/llvm/llvm-project/issues/165793
exposing the change from a vector fp-to-int conversion into a scalar one
inside the loop.
Additionally, removed
`llvm/test/Transforms/InstCombine/vec_extract_var_elt-inseltpoison.ll`,
which was a duplicate of
`llvm/test/Transforms/InstCombine/vec_extract_var_elt.ll` differing only
by the use of poison in the insert element.
The poison variant has been merged into
`llvm/test/Transforms/InstCombine/vec_extract_var_elt.ll`.
Also improved test naming and updated test checks.