[X86] Do not generate __multi3 for mul i128 on X86

Summary: __multi3 is not available on x86 (32-bit). Setting lib call name for MULI_128 to nullptr forces DAGTypeLegalizer::ExpandIntRes_MUL to generate instructions for 128-bit multiply instead of a call to an undefined function.  This fixes PR20871 though it may be worth looking at why licm and indvars combine to generate 65-bit multiplies in that test.

Patch by Riyaz V Puthiyapurayil

Reviewers: craig.topper, schweitz

Reviewed By: craig.topper, schweitz

Subscribers: RKSimon, llvm-commits

Differential Revision: https://reviews.llvm.org/D38668

llvm-svn: 316254