Never enregister type-punned regStruct lclVars.

Never enregister type-punned regStruct lclVars.

Type-punned lclVars--i.e. lclVars that are accessed via
`(INDIR Tx (ADDR (LCLVAR Ty V)))`--are currently considered
enregisterable if they are SIMD-typed, have not been the accessed by
field, and have not otherwise been declared unenregisterable. This
appears to be an oversight, as the JIT is not able to generate correct
code for the type-punned access if the lclVar is in fact enregistered.

This change removes the special case for these SIMD-typed lclVars.

Fixes dotnet/coreclr#11804.

Commit migrated from https://github.com/dotnet/coreclr/commit/e1bc678f35adb2d3f1ca40d0414bde2407159c2d