[Scalarizer] Fix out-of-bounds crash (#180359)
When processing an extractelement instruction with an index that exceeds
the vector size (e.g., extracting index 2147483647 from a 4-element
vector), the scalarizer would calculate an out-of-bounds Fragment index
and crash with an assertion failure in `SmallVector::operator[]`.
This PR adds a bounds check in
`ScalarizerVisitor::visitExtractElementInst` to prevent a crash when the
extractelement index is out of bounds.
Fixes #179880
