Load and store instructions, as well as intrinsics which access memory,
can now shared the same methods for accessing the memory address and
data operands.

Note that while this change introduces the potential for non-load/store
instructions to have their operands accessed through getLoadAddress(),
getStoreAddress(), or getData(), that risk isn't any greater than using
the wrong getSrc() index, and would stick out as a mistake much more
clearly. The advantage this change brings is that we no longer have to
remember where the address and data operands are stored in sub-vector
load/store intrinsics. In addition, there are no more overly verbose
casts, and their cost is eliminated.

Bug: b/179497998
Change-Id: I0d9208555e00b0d3053f7d3baca241fef2b8cbeb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52531
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>