It is no longer derived from InstCall, and doesn't take a Target
parameter which could be a symbol to a function which implements
the intrinsic.

Note one can still emit actual Call instructions if a function call
is needed. Since the previous change which removed the Target parameter
we can no longer decide between implementing an intrinsic as inline
instructions or a function call at the Subzero level, but we can still
do that at the Reactor level which has its own concept of intrinsics.

This change also removes mentions of intrinsics representing function
calls. It also removes code related to PNaCl-specific LLVM intrinsics,
including the ability to look up intrinsics by name. The addArg(),
getArg(), and getNumArgs() methods, adopted from InstCall (but no longer
inherited from it), are kept for now due to risk of replacing the ones
for InstCall objects, while the confusion caused by keeping the
function-related "arg" term is deemed low.

Bug: b/179497998
Change-Id: I293f039853abff6f5bebda1b714774205bdec846
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52608
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>