[Arm64] Extend Compiler::lvaFrameAddress() and JIT to allow using SP as base register

[echesakov]

At the moment, on Arm64 we always establish frame pointer when allocating a stack frame. Given that fact and based on the logic in Compiler::lvaFrameAddress() the JIT would always use fp as a base register for addressing locals and temps. This can lead to un-optimal codegen in a situation when the JIT is required to use frameType=5 (i.e., when fp points to a location above locals) instead of frameType=3 (i.e., when fp points to a location below locals). The following is a snippet of such un-optimal codegen when such restriction on fp value is imposed:

-;  V285 tmp284      [V285    ] (  2,  4   )  struct ( 8) [fp+0x1C50]   do-not-enreg[XSB] addr-exposed "Inlining Arg"
+;  V285 tmp284      [V285    ] (  2,  4   )  struct ( 8) [fp-0x7E8]   do-not-enreg[XSB] addr-exposed "Inlining Arg"

-;  TEMP_01                                   byref  ->  [fp+0x10]
+;  TEMP_01                                   byref  ->  [fp-0x2428]

- ldrh    w11, [fp,#0xd1ffab1e]      // [V285 tmp284]
+ movn    xip1, #0xd1ffab1e
+ ldrh    w11, [fp, xip1]    // [V285 tmp284]

- str     x14, [fp,#16]      // [TEMP_01]
+ movn    xip1, #0xd1ffab1e
+ str     x14, [fp, xip1]    // [TEMP_01]

I noticed this while implementing stack probe helper support on Arm64 in #43250 when the JIT is required to store fp,lr register pair earlier during prolog execution to be able to call a helper.

@dotnet/jit-contrib

category:cq
theme:register-allocator

[echesakov]

Un-assigning myself
cc @BruceForstall