gdb/record: add support for AVX conversion instructions.

WIP

This commit adds support for instructions to convert from one type to
another, which are in the form:
* VCVTDQ2[PS|PD]
* VCVTPS2[DQ|PD]
* VCVTPD2[PS|DQ]
* VCVTSD2[SI|SS]
* VCVTSI2[SS|SD]
* VCVTSS2[SD|SI]
* VCVTTP[S|D]2DQ
* VCVTTS[S|D]2SI

It also adds support to vpsadbw, since it was trivial and only one
instruction. Finally, I have slightly reorder the case statements to
keep them in numerical order.