LLVM claims to support atomic loads and stores of integers, pointers and floating point types, but certain backends cannot deal with floats.

Mimic what Clang does - bitcast to/from an equal width integer.

Bug: b/136037244
Change-Id: I40167279de96f586414f4fb406bf100432a39abf
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33408
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>