looprestoration: Make the C wiener h filter more optimizable for the compiler

This increases the binary size by 9 KB, on aarch64 with Xcode Clang 16,
if the C version of the filter is retained (which it isn't
by default).

This makes the vectorized C code roughly as fast as it was before
the rewrite on GCC; on Clang it also becomes 1.3x-2.0x faster,
while still being slower than it was initially.