[SelectionDAG][VP] Rename VP sext/zext/trunc ISD opcodes

Rather than VP_SEXT/VP_ZEXT/VP_TRUNC, having
VP_SIGN_EXTEND/VP_ZERO_EXTEND/VP_TRUNCATE better matches their non-VP
counterparts.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D125298