[LICM] Remove too conservative IsMustExecute variable

LICM relies on variable `MustExecute` which is conservatively set to `false`
in all non-headers. It is used when we decide whether or not we want to hoist
an instruction or a guard.

For the guards, it might be too conservative to use this variable, we can
instead use a more precise logic from LoopSafetyInfo. Currently it is only NFC
because `IsMemoryNotModified` is also conservatively set to `false` for all
non-headers, and we cannot hoist guards from non-header blocks. However once we
give up using `IsMemoryNotModified` and use a smarter check instead, this will
allow us to hoist guards from all mustexecute non-header blocks.

Differential Revision: https://reviews.llvm.org/D50888
Reveiwed By: fedor.sergeev

llvm-svn: 346204