Reland "[WebAssembly] Emulate v128.const efficiently""

This reverts commit 432e4e56, which reverted 542523a6. Two issues from
the original commit have been fixed. First, MSVC does not like when std::array
is initialized with only single braces, so this commit switches to using the
more portable double braces. Second, there was a subtle endianness bug that
prevented the original commit from working correctly on big-endian machines,
which has been fixed by switching to using endianness-agnostic bit twiddling
instead of type punning.

Differential Revision: https://reviews.llvm.org/D88773