Herbert Valerio Riedel authored Jan 26, 2018 and Ben Gamari committed Jan 29, 2018

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 31c260f3)