[llvm] SystemZ: Handle copies between gr64 and fp64 (PR #124890)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> ```
> // Moves between 64-bit integer and floating-point registers.
> def LGDR : UnaryRRE<"lgdr", 0xB3CD, bitconvert, GR64, FP64>;
> def LDGR : UnaryRRE<"ldgr", 0xB3C1, bitconvert, FP64, GR64>;
> ```
> 
> If we add those instructions to `copyPhysReg`, _should_ those bitcasts then just emit copies instead?

Probably. There's a bit of code scattered around to handle "bitcast" instructions, and I'm not really sure why we have it.

https://github.com/llvm/llvm-project/pull/124890