varasm: check float size [PR109359]

varasm: check float size [PR109359]

In PR95226, the testcase was failing because we tried to output_constant a
NOP_EXPR to float from a double REAL_CST, and so we output a double where
the caller wanted a float.  That doesn't happen anymore, but with the
output_constant hunk we will ICE in that situation rather than emit the
wrong number of bytes.

Part of the problem was that initializer_constant_valid_p_1 returned true
for that NOP_EXPR, because it compared the sizes of integer types but not
floating-point types.  So the C++ front end assumed it didn't need to fold
the initializer.

This also fixed the test for PR109359.

	PR c++/95226
	PR c++/109359

gcc/ChangeLog:

	* varasm.cc (output_constant) [REAL_TYPE]: Check that sizes match.
	(initializer_constant_valid_p_1): Compare float precision.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/frounding-math1.C: New test.

(cherry picked from commit e7cc4d703bceb9095316c106eba0d1939c6c8044)