[nvptx] fix complex args

[Nathan Sidwell]

I've committed this patch, which fixes gcc.dg/compat/struct-by-value-11

That test passes 'complex char', which is split into two char arguments.  We 
fail to promote the type in one place, leading to a PTX type mismatch where we 
claim an 8 bit type is a 32 bit type.

nathan

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