Revert ELG_LOP back to being its own target class for SV3

[geordie666]

@djschlegel and @araichoor pointed out that it was probably a bad idea to change the meaning of the ELG bit, as I did in #700. So, this PR reverts back to the old meanings for the ELGs. To be clear, as of now (and forevermore for the purposes of sv3):

• The bit ELG_LOP means a low-priority ELG.
• The bit ELG_HIP means a high-priority ELG.
• The bit ELG means either a low-priority ELG or a high-priority ELG (ELG is ELG_LOP | ELG_HIP).

To ensure the correct prioritization behavior, the MTL code (for sv3) now utilizes ELG_LOP and ELG_HIP and ignores ELG completely.

I'll run replacement targeting files soon and email desi-targets indicating the location of the new files.

[coveralls]

Coverage increased (+0.02%) to 60.648% when pulling 69486b6 on ADM-revert-ELG into c36a128 on master.