Don't promote byval pointer arguments when padding matters

Don't promote byval pointer arguments when when their size in bits is
not equal to their alloc size in bits. This can happen for x86_fp80,
where the size in bits is 80 but the alloca size in bits in 128.
Promoting these types can break passing unions of x86_fp80s and other
types.

Patch by Thomas Jablin!

Reviewed By: rnk

Differential Revision: http://reviews.llvm.org/D5057