i965: Don't compact instructions with unmapped bits.

i965: Don't compact instructions with unmapped bits.

Some instruction bits don't have a mapping defined to any compacted
instruction field.  If they're ever set and we end up compacting the
instruction they will be forced to zero.  Avoid using compaction in such
cases.

v2: Align multiple lines of an expression to the same column.  Change
    conditional compaction of 3-source instructions to an
    assertion. (Matt)
v3: The 3-source instruction bit 105 is part of SourceIndex on CHV.
    Add assertion that reserved bit 7 is not set. (Matt)
    Document overlap with UIP and 64-bit immediate fields.
v4: Make some more unmapped bit checks assertions. (Matt)

Reviewed-by: Matt Turner <mattst88@gmail.com>