The introduction of the sparse bitmap on-entry cache for large BB 
functions also introduced the concept that the value we ask to be 
written to the cache may not be properly represented.  It is limited to 
15 unique ranges for any given ssa-name, then  it reverts to varying for 
any additional values to be safe.

This patch adds a boolean return value to the cache set routines, 
allowing us to check if the value was properly written.

Other than that, no functional change.

Bootstrap on x86_64-pc-linux-gnu with no regressions.  pushed.

Andrew