x86: add a few more NOP patterns

First of all add f32_5[], allowing to eliminate the extra slot-is-NULL
code from i386_output_nops(). Plus then introduce f32_8[] and f16_5[]
following the same concept of adding a %cs segment override prefix.

Also re-use patterns when possible and correct comments as applicable.
Similarly re-use testcase expectations as much as possible, where they
need touching anyway.