Use canonicalize_math_after_vectorization_p for FMA folds

Use canonicalize_math_after_vectorization_p for FMA folds

The folds in r260348 kicked in before vectorisation, which hurts
for two reasons:

(1) the current suboptimal handling of nothrow meant that we could
    drop the flag early and so prevent if-conversion

(2) some architectures provide more scalar forms than vector forms
    (true for Advanced SIMD)

(1) is a bug in itself that needs to be fixed eventually, but delaying
the folds is still needed for (2).

2018-05-24  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* match.pd: Delay FMA folds until after vectorization.

gcc/testsuite/
	* gcc.dg/vect/vect-fma-1.c: New test.

From-SVN: r260639