[VE] LEGALAVL and staged VVP legalization

The new LEGALAVL node annotates that the AVL refers to packs of 64bit.
We use a two-stage lowering approach with LEGALAVL:

First, standard SDNodes are translated into illegal VVP layer nodes.
Regardless of source (VP or standard), all VVP nodes have a mask and AVL
parameter. The AVL parameter refers to the element position (just as in
VP intrinsics).

Second, we legalize the AVL usage in VVP layer nodes. If the element
size is < 64bit, the EVL parameter has to be adjusted to refer to packs
of 64bits.  We wrap the legalized AVL in a LEGALAVL node to track this.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D118321

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