Handle SIMD8/LONG recasts for LCL_FLD

[CarolEidt]

Lowering needs to insert a BITCAST in the case of a STORE_LCL_FLD with mismatched TYP_SIMD8/TYP_LONG operands, just as for STORE_LCL_VAR.

[briansull]

LGTM

[CarolEidt]

This test case exposed a different issue (also related to SIMD8) on linux/x64. In that case, we can have a promoted field that's TYP_SIMD8 (Vector2), to which we assign a TYP_DOUBLE because that's the type that a TYPE_SIMD8 is returned as.
The code in fgMorphLocalField() had an overly-aggressive assert that doesn't apply for targets that can return structs in fp regs and is not really very useful. Also, it was checking to see whether the type of the promoted field can be enregistered, but we know that it can - otherwise we don't promote it.
The fix for this is in the second commit.

[briansull]

Looks Good

[CarolEidt]

test Ubuntu arm Cross Checked Innerloop Build and Test