path: fix POSIX path.resolve() perf regression

[mscdex]

b0d5e03 introduced an unnecessary performance regression when using path.resolve() on POSIX systems. This commit both removes that regression in that situation and reduces the performance hit when path.posix.resolve() is used directly/indirectly on Windows systems.

The following benchmark results come from a Linux system:

After b0d5e03:

                                                                           confidence improvement accuracy (*)   (**)  (***)
path/resolve-posix.jsn=1000000 paths=''                                          ***    -54.27 %       ±0.94% ±1.26% ±1.65%
path/resolve-posix.jsn=1000000 paths='|'                                         ***    -54.97 %       ±1.07% ±1.43% ±1.86%
path/resolve-posix.jsn=1000000 paths='a/b/c/|../../..'                           ***    -34.18 %       ±0.86% ±1.15% ±1.50%
path/resolve-posix.jsn=1000000 paths='foo/bar|/tmp/file/|..|a/../subfile'        ***      8.11 %       ±1.55% ±2.07% ±2.71%

After the new implementation (basically caching the regexp):

                                                                           confidence improvement accuracy (*)   (**)  (***)
path/resolve-posix.jsn=1000000 paths=''                                          ***    -12.13 %       ±1.60% ±2.13% ±2.78%
path/resolve-posix.jsn=1000000 paths='|'                                         ***    -11.08 %       ±1.37% ±1.82% ±2.37%
path/resolve-posix.jsn=1000000 paths='a/b/c/|../../..'                           ***     -6.80 %       ±1.05% ±1.40% ±1.83%
path/resolve-posix.jsn=1000000 paths='foo/bar|/tmp/file/|..|a/../subfile'        ***      7.86 %       ±1.40% ±1.87% ±2.44%

After avoiding the transformations from b0d5e03 for POSIX systems (I'm not sure why there is a performance increase in this case, as I would have expected zero or close to zero change, but that's V8 for you):

                                                                           confidence improvement accuracy (*)   (**)  (***)
path/resolve-posix.jsn=1000000 paths=''                                          ***      9.13 %       ±1.16% ±1.54% ±2.01%
path/resolve-posix.jsn=1000000 paths='|'                                         ***      8.69 %       ±1.13% ±1.51% ±1.97%
path/resolve-posix.jsn=1000000 paths='a/b/c/|../../..'                           ***      4.82 %       ±0.91% ±1.21% ±1.58%
path/resolve-posix.jsn=1000000 paths='foo/bar|/tmp/file/|..|a/../subfile'        ***      8.62 %       ±1.24% ±1.66% ±2.17%

/cc @Trott @mcollina

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/37143/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/37152/

[Trott]

Fast-track to get this into the 15.14.0 release that's happening in another day-and-a-half or so?

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/37160/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/37165/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/37169/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/37173/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/37184/

[Trott]

Landed in e38d62a.