Use llvm::VectorType::getNumElements() instead by casting to the
expected subtype.

Bug: b/152339534
Change-Id: I29d2b8d60872f6f4eff4ae2bb38d1738515d11dc
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44068
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>