Fix llc crash processing S/UREM for -Oz builds caused by rL250825.

Fix llc crash processing S/UREM for -Oz builds caused by rL250825.

When taking the remainder of a value divided by a constant, visitREM()
attempts to convert the REM to a longer but faster sequence of instructions.
This conversion calls combine() on a speculative DIV instruction. Commit
rL250825 may cause this combine() to return a DIVREM, corrupting nearby nodes.
Flow eventually hits unreachable().

This patch adds a test case and a check to prevent visitREM() from trying
to convert the REM instruction in cases where a DIVREM is possible.
See http://reviews.llvm.org/D14035

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251373 91177308-0d34-0410-b5e6-96231b3b80d8