Implement bitcast between i32 and (emulated) v4i8.
Implement bitcast between i32 and (emulated) v4i8.

BUG=swiftshader:15

Change-Id: Ic795def8a914508ab0d850c846b73b343ace45de

bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader