Pull in r301983 from upstream llvm trunk (by Tim Northover):

Pull in r301983 from upstream llvm trunk (by Tim Northover):

  ARM: avoid handing a deleted node back to TableGen during ISel.

  When we replaced the multiplicand the destination node might already
  exist. When that happens the original gets CSEd and deleted. However,
  it's actually used as the offset so nonsense is produced.

  Should fix PR32726.

This fixes an assertion failure when building building www/firefox 53.0
for arm.

Reported by:	Bob Prohaska
PR:		218782
MFC after:	3 days