Remove the user-count threshold when analyzing read attributes

Summary:
This threshold limited FunctionAttrs ability to prove arguments to be read-only. 
In NVPTX, a specialized instruction ld.global.nc can be used to load memory
with non-coherent texture cache. We notice that in SHOC [1] benchmark, some
function arguments are not marked with readonly because FunctionAttrs reaches
a hardcoded threshold when analysis uses.

Removing this threshold won't cause significant regression in compilation time, because the worst-case time complexity of the algorithm is still O(# of instructions) for each parameter.

Patched by Xuetian Weng.  

[1] https://github.com/vetter/shoc

Reviewers: nlewycky, jingyue, nicholas

Subscribers: nicholas, test, llvm-commits

Differential Revision: http://reviews.llvm.org/D11311

llvm-svn: 243141