[VE] Add lvm/svm intrinsic instructions

Add lvm/svm intrinsic instructions and a regression test.  Change
RegisterInfo to specify that VM0/VMP0 are constant and reserved
registers.  This modifies a vst regression test, so update it.
Also add pseudo instructions for VM512 register classes
and mechanism to expand them after register allocation.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D91541