Fix MSB0 issues for SVP64

Fix MSB0 issues for SVP64

Main changes are:
1) Convert indices from MSB0 to LSB0 when extracting fields
2) Convert indices from LSB0 to MSB0 when inserting fields
3) Reorder nMigen Records to start from the LSB

This was verified by inspecting the GTKWave output for
test_issuer_svp64.py, checking the instruction memory against a manually
assembled instruction, and checking that the decoded fields correspond to
the original instruction.