[VE] LVLGen sets VL before vector insts

The VE backend represents vector instructions with an explicit 'i32'
vector length operand.  In the VE ISA, the vector length is always read
from the VL hardware register.  The LVLGen pass inserts 'lvl'
instructions as necessary to set VL to the right value before each
vector instruction.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D91416