[PATCH] Allow BB duplication threshold to be adjusted through JumpThreading's ctor

[Michael Liao]

Hi Hal

Yeah, "noduplicate" could prevent duplicating of barrier calls but that 
patch wants to address the potential issue on processors with divergent 
control flow, commonly found in GPUs, e.g. AMD/NVIDIA ones. The 
scenario is that, if BB is duplicated to exploit more jump threading, 
targets with divergent CF may execute more instructions if the 
condition is a divergent one.

For updating that threshold from TTI, yeah, if we are interested in 
that case. I could come another patch considering both TTI and 
user-specified threshold.

Yours
- Michael

http://reviews.llvm.org/D5444