[WebAssembly] SIMD extract_lane

Implement instruction selection for all versions of the extract_lane
instruction. Use explicit sext/zext to differentiate between
extract_lane_s and extract_lane_u for applicable types, otherwise
default to extract_lane_u.

Reviewers: aheejin

Subscribers: sunfish, jgravelle-google, sbc100, llvm-commits

Differential Revision: https://reviews.llvm.org/D50597

Patch by Thomas Lively (tlively)

llvm-svn: 339707