[cherry-pick] [MIPS] Initial support for MIPS-I load delay slots

[ayrtonm]

There's been interest in adding a target for the Sony PlayStation 1 so this cherry picks the commit adding initial support for MIPS-I since LLVM was previously generating incorrect code. Both the github issue and LLVM review in the original commit message below list remaining tasks for MIPS-I support, but load delay slots were the main issue for the PS1.

LLVM so far has only supported the MIPS-II and above architectures. MIPS-II is pretty close to MIPS-I, the major difference
being that "load" instructions always take one extra instruction slot to propogate to registers. This patch adds support for
MIPS-I by adding hazard handling for load delay slots, alongside MIPSR6 forbidden slots and FPU slots, inserting a NOP
instruction between a load and any instruction immediately following that reads the load's destination register. I also
included a simple regression test. Since no existing tests target MIPS-I, those all still pass.

Issue ref: simias/psx-sdk-rs#1

I also tested by building a simple demo app with Clang and running it in an emulator.

Patch by: @impiaaa

Differential Revision: https://reviews.llvm.org/D122427