Add ptr-eq short-cut to `compareByteArrays#` primitive

Authored by hvr on Jan 26 2018, 12:07 PM.

Description

Add ptr-eq short-cut to compareByteArrays# primitive

This is an obvious optimisation whose overhead is neglectable but
which significantly simplifies the common uses of compareByteArrays#
which would otherwise require to make *careful* use of
reallyUnsafePtrEquality# or (equally fragile) byteArrayContents#
which can result in less optimal assembler code being generated.

Test Plan: carefully examined generated cmm/asm code; validate via phab

Reviewers: alexbiehl, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4319

(cherry picked from commit 31c260f3967d2c06063c962a98475058daa45c6d)