x86: honor template rather than actual operands when updating i.xstate

This undoes a change to md_assemble() that 32930e4e ("x86: Support
GNU_PROPERTY_X86_ISA_1_V[234] marker") did without any explanation. This
broke a CVTPI2PS property test that a subsequent test will add, and the
updates to existing tests also demonstrate what was wrong: For example,
AVX insns update the full YMM, even if a Vex128 variant is in use.