Implement some constexpr vector unary operators, fix boolean-ops

As requested in the review, this implements unary +,-,~, and ! for
vector types.

All of our boolean operations on vector types should be using something
like vcmpeqd, which results in a mask of '-1' for the 'truth' type. We are
currently instead using '1', which results in some incorrect
calculations when used later (note that it does NOT result in a boolean
vector, as that is not really a thing).

This patch corrects that 1 to be a -1, and updates the affected tests.

Differential Revision: https://reviews.llvm.org/D115670