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[musttail] Don't forward AL on Win64

Authored by rnk on Nov 18 2019, 3:31 PM.

Description

[musttail] Don't forward AL on Win64

AL is only used for varargs on SysV platforms. Don't forward it on
Windows. This allows control flow guard to set up an extra hidden
parameter in RAX, as described in PR44049.

This also has the effect of freeing up RAX for use in virtual member
pointer thunks, which may also be a nice little code size improvement on
Win64.

Fixes PR44049

Reviewers: ajpaverd, efriedma, hans

Differential Revision: https://reviews.llvm.org/D70413