Remove unnecessary branch in Range::find_first_of(Range,size_t)

Remove unnecessary branch in Range::find_first_of(Range,size_t)

Summary:
Added benchmarks.

Before:

folly/test/RangeFindBenchmark.cpp               relative  time/iter  iters/s
============================================================================
string length 10:
FindFirstOfRange                                             1.36ns  733.07M
FindFirstOfOffsetRange                                       2.15ns  464.16M
============================================================================
string length 256:
FindFirstOfRange                                             1.36ns  733.07M
FindFirstOfOffsetRange                                       1.42ns  704.22M
============================================================================
string length 10240:
FindFirstOfRange                                             1.36ns  733.07M
FindFirstOfOffsetRange                                       3.72ns  268.70M
============================================================================
string length 10485760:
FindFirstOfRange                                             1.36ns  733.07M
FindFirstOfOffsetRange                                       5.00ns  199.96M

After:

string length 10:
FindFirstOfRange                                             1.36ns  732.92M
FindFirstOfOffsetRange                                       1.36ns  732.61M
============================================================================
string length 256:
FindFirstOfRange                                             1.36ns  732.93M
FindFirstOfOffsetRange                                       1.38ns  727.16M
============================================================================
string length 10240:
FindFirstOfRange                                             1.36ns  732.93M
FindFirstOfOffsetRange                                       1.79ns  558.40M
============================================================================
string length 10485760:
FindFirstOfRange                                             1.36ns  732.93M
FindFirstOfOffsetRange                                       2.73ns  366.44M

Test Plan: fbconfig folly && fbmake runtests

Reviewed By: delong.j@fb.com

FB internal diff: D660125