Test case for PR rtl-optimization/60473

Test case for PR rtl-optimization/60473

PR rtl-optimization/60473 is code quality regression that has
been cured by improvements to register allocation.  For the function
in the test case, GCC 4.4, 4.5 and 4.6 generated very poor code
requiring two mov instructions, and GCC 4.7 and 4.8 (when the PR was
filed) produced better but still poor code with one mov instruction.
Since GCC 4.9 (including current mainline), it generates optimal
code with no mov instructions, matching what used to be generated
in GCC 4.1.

2020-08-04  Roger Sayle  <roger@nextmovesoftware.com>

gcc/testsuite/ChangeLog
	PR rtl-optimization/60473
	* gcc.target/i386/pr60473.c: New test.