[Libre-soc-dev] cray-style vector of 40 years setting VL=0 at runtime

[lkcl]

first encounters of areas that result in behavioural changes:

* predication without zeroing (first encountered bit set, which may be NO bit set in the case of scalar)
* REMAP
* LD/ST which uses "RA.isvec" (and RB.isvec) to set modes.

REMAP can be "solved" by using RT.scalar and at least one of SRC.isvec=1, it achieves the same goal.

however LD/ST- imm/idx both need more comprehensive analysis.