libstdc++: Improve simd fixed_size codegen

libstdc++: Improve simd fixed_size codegen

Sometimes fixed_size objects will get unnecessarily copied on the stack.
The simd implementation should never pass _SimdTuple by value to avoid
requiring the optimizer to see through these copies.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd_converter.h
	(_SimdConverter::operator()): Pass _SimdTuple by const-ref.
	* include/experimental/bits/simd_fixed_size.h
	(_GLIBCXX_SIMD_FIXED_OP): Pass binary operator _SimdTuple
	arguments by const-ref.
	(_S_masked_unary): Pass _SimdTuple by const-ref.