[ARM] Better AND's for MVE compares

Add a number of folds to convert and(vcmp, vcmp) into a single VPT block, where
the second vcmp becomes predicated on the first.

The VCMP; VPST; VCMP will eventually be converted to VPT; VCMP in the
VPTBlockPass.

Differential Revision: https://reviews.llvm.org/D65058

llvm-svn: 366910