Make musttail more robust for vector types on x86

Previously I tried to plug musttail into the existing vararg lowering
code. That turned out to be a mistake, because non-vararg calls use
significantly different register lowering, even on x86. For example, AVX
vectors are usually passed in registers to normal functions and memory
to vararg functions.  Now musttail uses a completely separate lowering.

Hopefully this can be used as the basis for non-x86 perfect forwarding.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D6156