[X86] AVX512: Factor generating the AsmString into avx512_icmp_cc

Adding a writemask variant would require a third asm string to be passed to
the template.  Generate the AsmString in the template instead.

No change in X86.td.expanded.
[X86] AVX512: Allow writemasks with vpcmp

For now I only updated the _alt variants.  The main variants are used by
codegen and that will need a bit more work to trigger.

<rdar://problem/17492620>