Correct Vectorcall Register passing and HVA Behavior

Front end component (back end changes are D27392).  The vectorcall 
calling convention was broken subtly in two cases.  First, 
it didn't properly handle homogeneous vector aggregates (HVAs). 
Second, the vectorcall specification requires that only the 
first 6 parameters be eligible for register assignment. 
This patch fixes both issues.

Differential Revision: https://reviews.llvm.org/D27529

llvm-svn: 291041