[WebAssembly] Fix a regression selecting negative build_vector lanes

Summary:
The custom lowering introduced in rL352592 creates build_vector nodes
with negative i32 operands, but these operands did not meet the value
range constraints necessary to match build_vector nodes. This CL fixes
the issue by removing the unnecessary constraints.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish

Differential Revision: https://reviews.llvm.org/D57481

llvm-svn: 352813