Fix compiler crash in BCE.

Remove a bogus call to HInstruction::AsConstant() that
causes a crash if the instruction is not a constant.

Test: New test in 449-checker-bce-rem
Test: testrunner.py --host --optimizing -t 449-checker-bce-rem
Bug: 169669115
Change-Id: I7f962441b652538b2d1cc4238eb9ab66abc1286c
2 files changed