Load/Store scalar accesses before/after the statement itself

Instead of generating implicit loads within basic blocks, put them 
before the instructions of the statment itself, including non-affine 
subregions. The region's entry node is dominating all blocks in the 
region and therefore the loaded value will be available there.

Implicit writes in block-stmts were already stored back at the end of 
the block. Now, also generate the stores of non-affine subregions when 
leaving the statement, i.e. in the exiting block.

This change is required for array-mapped implicits ("De-LICM") to 
ensure that there are no dependencies of demoted scalars within 
statments. Statement load all required values, operator on copied in 
registers, and then write back the changed value to the demoted memory. 
Lifetimes analysis within statements becomes unecessary.

Differential Revision: http://reviews.llvm.org/D13487

llvm-svn: 250625