LSR: Calculate instruction cost only if InsnsCost is set to true (NFC)

Summary:

The patch guard all instruction cost calculations with InsnCosts (-lsr-insns-cost) option.
Currently even if the option set to false we calculate and print (in debug mode) instruction costs.

Reviewers: qcolombet

Differential Revision: http://reviews.llvm.org/D33914

From: Evgeny Stupachenko <evstupac@gmail.com>
llvm-svn: 304746