Add early exit for argsort if array is already sorted

[r-devulap]

Relative to scalar argsort, SIMD argsort is slower for ordered/sorted arrays. An easy fix is to detect sorted arrays and exit early. This doesn't add much overhead to random arrays but does slightly regress on uniform/constant arrays. But perhaps that trade off is worth it since argsort is fairly faster than scalar argsort for uniform/constant arrays. Benchmarks on AVX2:

Benchmark                                                                                   Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------------------------------------
[simdargsort/constant_10k/uint64_t vs. simdargsort/constant_10k/uint64_t]                +0.1902         +0.1902         14544         17310         14544         17310
[simdargsort/sorted_10k/uint64_t vs. simdargsort/sorted_10k/uint64_t]                -0.9408         -0.9408        292847         17329        292836         17329

[sterrettm2]

Looks good to me!