PR c/82071 reports how compound assignment operators such as += handle
excess precision inconsistently with the same operation done with a
plain assignment and binary operator.

There were (at least) two problems with how compound assignments
handled excess precision.  The EXCESS_PRECISION_EXPR for an argument
with excess precision was removed too early, resulting in
build_binary_op being called with an rhs operand whose type reflected
the evaluation format, so not having sufficient information to achieve
the intended semantics in all cases, and then the code called
c_fully_fold on the results of build_binary_op without allowing for
the possibility of an EXCESS_PRECISION_EXPR as the result, so leading
to double rounding of the result (first to its semantic type, then to
the type of the LHS of the assignment) instead of the intended single
rounding.

This patch fixes those problems by keeping EXCESS_PRECISION_EXPRs
further through build_modify_expr (and build_atomic_assign which it
calls) and only removing them locally where appropriate.

Note that while this patch should achieve *consistency*, that's
consistency with the understanding of C99 semantics that I originally
intended to implement.  For the particular case in the testcase, C11
semantics (from N1531) differ from that understanding of C99
semantics, in that an implicit conversion of an integer to floating
point can have excess precision.  I intend to implement those C11
semantics separately (conditional on flag_isoc11) (which will also
mean that building conditional expressions can produce a result with
excess precision even when the arguments lack excess precision, where
previously it could not), and not to close the bug until that is also
done.

Tested for x86_64-pc-linux-gnu.

	PR c/82071
gcc/c:
	* c-typeck.c (build_atomic_assign): Handle argument with excess
	precision.  Ensure any EXCESS_PRECISION_EXPR is present in
	argument passed to build_binary_op and convert_for_assignment but
	not for call to c_fully_fold.
	(build_modify_expr): Do not remove EXCESS_PRECISION_EXPR early.
	Ensure build_binary_op is called with argument with original
	semantic type.  Avoid calling c_fully_fold with an
	EXCESS_PRECISION_EXPR from build_binary_op.

gcc/testsuite:
	* gcc.target/i386/excess-precision-7.c: New test.

From-SVN: r251603