[Target][ARM] Change VPTMaskValues to the correct encoding

VPTMaskValue was using the "instruction" encoding to represent the masks
(= the same encoding as the one used by the instructions in an object file),
but it is only used to build MCOperands, so it should use the MCOperand
encoding of the masks, which is slightly different.

Differential Revision: https://reviews.llvm.org/D76139