Fix FileSpec to be able to understand Windows paths.

This patch adds the notion of a "path syntax" to FileSpec.  There
are two syntaxes (Posix and Windows) and one "meta syntax",
Host Native, which uses the current platform to figure out the
appropriate syntax for host paths.

This allows paths from one platform to be represented and
manipulated on another platform even if they have different path
syntaxes.
Optimizations for FileSpec.
[AVX512] Generate masking instruction variants with tablegen

After adding the masking variants to several instructions, I have decided to
experiment with generating these from the non-masking/unconditional
variant. This will hopefully reduce the amount repetition that we currently
have in order to define an instruction with all its variants (for a reg/mem
instruction this would be 6 instruction defs and 2 Pat<> for the intrinsic).

The patch is the first cut that is currently only applied to valignd/q to make
the patch small.

A few notes on the approach:

  * In order to stitch together the dag for both the conditional and the
  unconditional patterns I pass the RHS of the set rather than the full
  pattern (set dest, RHS).
  * Rather than subclassing each instruction base class (e.g. AVX512AIi8),
  with a masking variant which wouldn't scale, I derived the masking
  instructions from a new base class AVX512 (this is just I<> with
  Requires<HasAVX512>).  The instructions derive from this now, plus a new set
  of classes that add the format bits and everything else that instruction
  base class provided (i.e. AVX512AIi8 vs. AVX512AIi8Base).

I hope we can go incrementally from here.  I expect that:

  * We will need different variants of the masking class.  One example is
  instructions requiring three vector sources.  In this case we tie one of the
  source operands to dest rather than a new implicit source operand ($src0)
  * Add the zero-masking variant
  * Add more AVX512*Base classes as new uses are added

I've looked at X86.td.expanded before and after to make sure that nothing got
lost for valignd/q.