PR rtl-optimization 61494: Preserve x-0.0 with HONOR_SNANS.

PR rtl-optimization 61494: Preserve x-0.0 with HONOR_SNANS.

The following patch avoids simplifying x-0.0 to x when -fsignaling-nans
is specified, which resolves PR rtl-optimization 61494.  Indeed, running
the test program attached to that PR now reports no failures.

2020-08-02  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/61494
	* simplify-rtx.c (simplify_binary_operation_1) [MINUS]: Don't
	simplify x - 0.0 with -fsignaling-nans.