[Hexagon] Use 'vnot' instead of 'not' in patterns with vectors

'not' expands to checking for an xor with a -1 constant. Since
this looks for a ConstantSDNode it will never match for a vector.

Co-authored-by: Craig Topper <craig.topper@sifive.com>

Differential Revision: https://reviews.llvm.org/D100687