[Bug rtl-optimization/115912] [15 regression] Harfbuzz testsuite fails (mvar_partial_instance test) since r15-1901-g98914f9eba5f19

[sjames at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115912

--- Comment #6 from Sam James <sjames at gcc dot gnu.org> ---
There's only a handful of notable looking differences:
```
│ -     and    $0xfffd,%ecx
│ +     and    $0xfffffffd,%edx
│ +     or     $0x2,%ecx
│       cmpb   $0x0,0xac8(%rsi)
│ -     je     144f0 <bool _subset<OT::glyf const>(hb_subset_plan_t*,
hb_vector_t<char, false>&)+0x1900>
│ -     or     $0x2,%edx
│ -     movzwl %dx,%ecx
│ -     movbe  %cx,0x10(%rax)
│ +     cmovne %ecx,%edx
│ +     movbe  %dx,0x10(%rax)
│       mov    0x78(%rsp),%rax
│       mov    %r13,%rdx
│       mov    $0x68656164,%esi
│       mov    0x50(%rax),%rdi
```

```
│ -     mov    0x20(%rsp),%r9d
│ +     mov    0x20(%rsp),%ecx
│       vroundss $0x9,%xmm0,%xmm0,%xmm0
│       vroundss $0x9,%xmm5,%xmm5,%xmm5
│       vcvttss2si %xmm0,%edx
│ -     movzwl %dx,%edx
│ -     vcvttss2si %xmm5,%ecx
│ -     movzwl %cx,%ecx
│ -     movswl %cx,%eax
│ +     vcvttss2si %xmm5,%r9d
│ +     movswl %r9w,%eax
│       vcvtsi2ss %eax,%xmm2,%xmm0
│       vaddss %xmm4,%xmm0,%xmm0
```

```
│       vmovdqu %ymm0,-0x70(%rbp)
│       vmovq  -0x250(%rbp),%xmm0
│ -     jmp    5c6
<OT::Layout::GPOS_impl::SinglePosFormat2::subset(hb_subset_context_t*)
const+0x5c6>
│ -     data16 cs nopw 0x0(%rax,%rax,1)
│ -     nopl   0x0(%rax)
│ +     jmp    5b6
<OT::Layout::GPOS_impl::SinglePosFormat2::subset(hb_subset_context_t*)
const+0x5b6>
│ +     xchg   %ax,%ax
│       movbe  0x2(%rcx),%si
```