[TargetLowering] Increase the storage size of NumRegistersForVT to allow the...

[TargetLowering] Increase the storage size of NumRegistersForVT to allow the type break down for v256i1 and other types to be stored correctly

v256i1 on X86 without avx512 breaks down to 256 i8 values when passed between basic blocks. But the NumRegistersForVT was sized at a byte for each VT. This results in 256 being stored as 0.

This patch enlarges the type to 16 bits and adds an assert to ensure that no information is lost when the entry is stored.

Differential Revision: https://reviews.llvm.org/D70138