[VP] llvm.vp.merge intrinsic and LangRef

llvm.vp.merge interprets the %evl operand differently than the other vp
intrinsics: all lanes at positions greater or equal than the %evl
operand are passed through from the second vector input. Otherwise it
behaves like llvm.vp.select.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D116725