Code size micro-optimizations in the X86 backend

Summary:
Carter Schonwald suggested looking for opportunities to replace
instructions in GHC's output by equivalent ones that are shorter,
as recommended by the Intel optimization manuals.

This patch reduces the module sizes as reported by nofib
by about 1.5% on x86_64.

Test Plan:
Built an i386 cross-compiler and ran the test suite; the same
(rather large) set of tests failed before and after this commit.
Will let Harbormaster validate on x86_64.

Reviewers: austin

Subscribers: thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D320