[PowerPC] Reduce stack frame for fastcc functions by only allocating parameter...

[PowerPC] Reduce stack frame for fastcc functions by only allocating parameter save area when needed

Current implementation always allocates the parameter save area conservatively
for fastcc functions. There is no reason to allocate the parameter save area if
all the parameters can be passed via registers.

Differential Revision: https://reviews.llvm.org/D42602

llvm-svn: 325581