[SVE] Remove getNumElements() warnings in InstCombiner::visitBitCast

Whilst trying to compile this test to assembly:

  CodeGen/aarch64-sve-intrinsics/acle_sve_reinterpret.c

I discovered some warnings were firing in InstCombiner::visitBitCast
due to calls to getNumElements() for scalable vector types. These
calls only really made sense for fixed width vectors so I have fixed
up the code appropriately.

Differential Revision: https://reviews.llvm.org/D80559