Sometimes (rarely) nodes held in LegalizeTypes

maps can be deleted.  This happens when RAUW
replaces a node N with another equivalent node
E, deleting the first node.  Solve this by
adding (N, E) to ReplacedNodes, which is already
used to remap nodes to replacements.  This means
that deleted nodes are being allowed in maps,
which can be delicate: the memory may be reused
for a new node which might get confused with the
old deleted node pointer hanging around in the
maps, so detect this and flush out maps if it
occurs (ExpungeNode).  The expunging operation
is expensive, however it never occurs during
a llvm-gcc bootstrap or anywhere in the nightly
testsuite.  It occurs three times in "make check":
Alpha/illegal-element-type.ll,
PowerPC/illegal-element-type.ll and
X86/mmx-shift.ll.  If expunging proves to be too
expensive then there are other more complicated
ways of solving the problem.
In the normal case this patch adds the overhead
of a few more map lookups, which is hopefully
negligable.

llvm-svn: 52214