[ARM] Allow signed icmps in ARMCodeGenPrepare

    
Originally committed in r339755 which was reverted in r339806 due to
an asan issue. The issue was caused by my assumption that operands to
a CallInst mapped to the FunctionType Params. CallInsts are now
handled by iterating over their ArgOperands instead of Operands.
    
Original Message:
  Treat signed icmps as 'sinks', allowing them to be in the use-def
  tree, enabling more promotions to be performed. As a sink, any
  promoted incoming values need to be truncated before being used by
  the signed icmp.
    
  Differential Revision: https://reviews.llvm.org/D50067

llvm-svn: 339858