[X86] Use GetDemandedBits to simplify the operands of PMULDQ/PMULUDQ.

This is an alternative to what I attempted in D56057.

GetDemandedBits is a special version of SimplifyDemandedBits that allows simplifications even when the operand has other uses. GetDemandedBits will only do simplifications that allow a node to be bypassed. It won't create new nodes or alter any of the other users.

I had to add support for bypassing SIGN_EXTEND_INREG to GetDemandedBits.

Based on a patch that Simon Pilgrim sent me in email.

Fixes PR40142.

llvm-svn: 350059