Avoid unnecessay .s2a write access when used only in PHIs

Accesses for exit node phis will be handled separately by 
buildPHIAccesses if there is more than one exiting edge, 
buildScalarDependences does not need to create additional SCALAR 
accesses.

This is a corrected version of r250517, which was reverted in r250607.

Differential Revision: http://reviews.llvm.org/D13848

llvm-svn: 250622