VectorType::getNumElements() will be removed in LLVM 12. Only
FixedVectorType will support this method. Cast to the latter where
appropriate.

Since LLVM 10 does not support FixedVectorType, treat it as an alias
for the legacy VectorType implementation.

Bug: b/354893276
Change-Id: Ie8160478819ff1c1bcd7fbe4425d2264869770e9
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52308Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>