[VP] Declaration and docs for vp.select intrinsic

llvm.vp.select extends the regular select instruction with an explicit
vector length (%evl).

All lanes with indexes at and above %evl are
undefined. Lanes below %evl are taken from the first input where the
mask is true and from the second input otherwise.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D105351