[ARM] MVE VMULL patterns

[ARM] MVE VMULL patterns

This adds MVE vmull patterns, which are conceptually the same as
mul(vmovl, vmovl), and so the tablegen patterns follow the same
structure.

For i8 and i16 this is simple enough, but in the i32 version the
multiply (in 64bits) is illegal, meaning we need to catch the pattern
earlier in a dag fold. Because bitcasts are involved in the zext
versions and the patterns are a little different in little and big
endian. I have only added little endian support in this patch.

Differential Revision: https://reviews.llvm.org/D76740