Replace deprecated VectorType::getNumElements with new APIs

[TH3CHARLie]

VectorType::getNumElements is first marked as deprecated (https://reviews.llvm.org/rG867de151a52b6d0750485ac1cf9b3bc012ee51fd) and is now deleted from LLVM's main branch (https://reviews.llvm.org/rG49a6502cd5c2361d9c6f49ae1ee36940afbdeb0a). So this PR replaces VectorType::getNumElements with newer APIs, following existing practice in cgutils.cpp:

julia/src/cgutils.cpp

Lines 1372 to 1377 in f442e42

	#if JL_LLVM_VERSION >= 120000
	ElementCount EC = VT->getElementCount();
	num_elements = EC.getKnownMinValue();
	#else
	num_elements = VT->getNumElements();
	#endif

[TH3CHARLie]

cc @vtjnash @aviatesk