[VE] Correct VMP allocation in calling conv

VE used to allocate VM1, VM2, VMP2 (VM4+VM5), and VM3.  This patch
corrects to allocate VM1, VM2, VMP2 (VM4+VM5), and VM6.  Also add
a regression test.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D93570