[GuardWidening] Make sure widened condition operands are available at insertion point

This fixes a possible issue when we could hoist an instruction up to a widenable
condition intrinsic call without making sure its operands are available at
hoisiting point.

Recently insertion point finding algorithm changed a bit, so this availability
check became necessary.

Verifier would crash after we handled the following special case:
  L >u C0 && L >u C1  ->  L >u max(C0, C1),
Previously we would insert the new condition right before the widenable condition
branch where all L operands were available.
Now we may choose the widenable condition intrinsic call as insertion point and it may
happen so that the L operands are computed after the call, so we have to make sure that
L operands are available at the point we want to insert it.

Differential Revision: https://reviews.llvm.org/D144944