[ARM] Make sure that the constant pool does not keep in the middle of an IT block.

[ARM] Make sure that the constant pool does not keep in the middle of an IT block.

This change make sure that llvm does not emit an invalid IT block
by putting the constant pool in the middle of an IT block.

We have code to try to avoid putting a constant island in the middle of an
IT block, but it only works if we see an IT between the one currently
referencing CPE and possible insertion point. If the first instruction
we look at is the VLDRD after the IT , we never see the IT and does not
realize that the instruction doing the load could be in an IT block itself.

Differential Revision: https://reviews.llvm.org/D64621

Change-Id: I24cecb37cded75e8992870bd997f6226853bd920
llvm-svn: 366905