Simplify RAV isSameMethod with constexpr if

Owing to the large number of instantiations of this function, this small
change has a small but meaningful difference on the total size of
(especially a debug) build of clang at -O0:
```
    FILE SIZE        VM SIZE
 --------------  --------------
  +0.9% +96.9Ki  +0.9% +96.9Ki    .data.rel.ro
  +0.7% +96.7Ki  +0.7% +96.7Ki    .rela.dyn
  +0.0% +18.3Ki  +0.0% +18.3Ki    .rodata
  +0.0%    +324  [ = ]       0    [2 Others]
  -0.2%    -392  -0.2%    -392    .gnu.version
  -0.0%    -441  [ = ]       0    .debug_abbrev
  -0.1%    -980  -0.1%    -980    .gnu.hash
  -0.2% -1.53Ki  -0.2% -1.53Ki    .hash
  -0.2% -4.59Ki  -0.2% -4.59Ki    .dynsym
  -0.1% -10.5Ki  [ = ]       0    .debug_rnglists
  -0.6% -59.0Ki  -0.6% -59.0Ki    .dynstr
  -0.2%  -191Ki  [ = ]       0    .debug_str_offsets
  -3.0%  -233Ki  -3.0%  -233Ki    .eh_frame_hdr
  -0.7%  -244Ki  [ = ]       0    .debug_addr
  -2.9%  -699Ki  [ = ]       0    .symtab
  -0.6%  -884Ki  [ = ]       0    .debug_line
  -3.0%  -932Ki  -3.0%  -932Ki    .eh_frame
  -1.0% -1.48Mi  -1.0% -1.48Mi    .text
  -0.6% -2.75Mi  [ = ]       0    .debug_info
  -7.3% -8.61Mi  [ = ]       0    .strtab
  -7.3% -17.2Mi  [ = ]       0    .debug_str
  -2.4% -33.0Mi  -0.9% -2.47Mi    TOTAL
```

If anyone's got other ideas for how to reduce this further - it's not
especially important, I just came across it while investigating a debug
info size regression, but thought it was interesting enough to poke
around at.