[WebAssembly] Initial SIMD128 support.

Kicks off the implementation of wasm SIMD128 support (spec:
https://github.com/stoklund/portable-simd/blob/master/portable-simd.md),
adding support for add, sub, mul for i8x16, i16x8, i32x4, and f32x4.

The spec is WIP, and might change in the near future.

Patch by João Porto

Differential Revision: https://reviews.llvm.org/D22686

llvm-svn: 277543
diff --git a/llvm/lib/Target/WebAssembly/WebAssembly.td b/llvm/lib/Target/WebAssembly/WebAssembly.td
index 551ad93..f647349 100644
--- a/llvm/lib/Target/WebAssembly/WebAssembly.td
+++ b/llvm/lib/Target/WebAssembly/WebAssembly.td
@@ -23,7 +23,7 @@
 // WebAssembly Subtarget features.
 //===----------------------------------------------------------------------===//
 
-def FeatureSIMD128 : SubtargetFeature<"simd128", "HasSIMD128", "false",
+def FeatureSIMD128 : SubtargetFeature<"simd128", "HasSIMD128", "true",
                                       "Enable 128-bit SIMD">;
 
 //===----------------------------------------------------------------------===//